What particles are thought to have existed before Inflation? What fundamental particles do most Grand Unified and Inflationary theories predict existed before the Inflationary period? Basically, what do we expect the family of particles existing during the Grand Unification epoch to look like? It is my understanding that before electroweak symmetry breaking, particles had no mass. Some sources also say they had no electric charge. Would there still be 3 generations of leptons and quarks then? Are electrons and neutrinos even distinguishable? I know that photons and WZ bosons recombine into electroweak bosons, but before inflation do those recombine with gluons? Is that what XY bosons are? Where can I find a straightforward answer to what a non-symmetry broken standard model looks like, as a lot of what I see seems contradictory?
 A: I think your question is about electroweak symmetry breaking and GUT symmetry breaking, and not inflation as such.
Here's a list of the fundamentally different particles in the Standard Model. There's one line for each family of particles that are related by Standard Model symmetries.

*

*U(1) field (2 degrees of freedom, usually seen as 1 boson with 2 spin states)

*SU(2) field (6 d.o.f., 3 bosons)

*SU(3) field (16 d.o.f., 8 bosons)

*3 copies/"generations" of each of the following (okay, I lied about one line for each; I didn't want to copy and paste):

*

*SU(2) and SU(3) charged Weyl fermions (12 d.o.f. = 2 spin states × 3 SU(3) colors × 2 SU(2) "colors")

*SU(3)-charged, SU(2)-uncharged Weyl fermions (6 d.o.f.)

*another SU(3)-charged, SU(2)-uncharged family with different U(1) charge (6 d.o.f.)

*SU(2)-charged, SU(3)-uncharged Weyl fermions (4 d.o.f.)

*U(1)-charged, SU(2)-uncharged, SU(3)-uncharged Weyl fermions (2 d.o.f.)

*Weyl fermions with no charge at all (2 d.o.f.) if you're including those to explain neutrino mass



*Higgs field (4 d.o.f.)

The fields after EWSB are the same; they are just "reinterpreted". As an analogy, if you have a wooden board that you initially covered with Cartesian coordinates, and the Texas sharpshooter of legend shoots a hole in the board, you may wish to switch to polar coordinates centered on the location of the hole, since they better respect the symmetry of the modified board. But the number of coordinates (2) is unchanged. If you had foreknowledge of where the hole would appear, then you could use the polar coordinates from the beginning. Presentations of the Standard Model tend to do that, which is why you'll sometimes hear that there are Higgs fields with different electric charges, even though those fields only make sense before EWSB and electric charge only makes sense after.
Post-EWSB, the up-type quarks are half of the SU(2)- and SU(3)-charged family of fermions coupled with one of the 6-d.o.f. families via the Higgs field, and the down-type quarks are the other half coupled with the other 6-d.o.f. family. The electrons and neutrinos work the same way if the uncharged fermions exist. If they don't exist then the neutrinos are just one half of the SU(2)-only family, not Higgs-coupled to anything.
The idea of grand unified theories is that something similar to EWSB happened at higher energy, leaving 12 massless bosons in three families (the U(1), SU(2) and SU(3) bosons), and some massive bosons like the W and Z, but much heavier, which are called X and sometimes Y.
One popular GUT is SU(5), which has a single SU(5) force with 24 gauge bosons / 48 d.o.f. (in general, SU(n) has $n^2-1$ gauge bosons). Instead of 5×3 charged fermion families, there are just 2×3 (while the uncharged fermions, if present, are still in their own 3 families).
Another popular GUT is SO(10), which has $10(10-1)/2=45$ bosons / 90 d.o.f., but has the advantage that all of the fermions in each generation, including the uncharged ones, are in a single family.

Are electrons and neutrinos even distinguishable?

The SU(2) charged halves of the electrons and neutrinos are aspects of the same thing pre-EWSB. The SU(2)-uncharged halves are different even pre-EWSB because they have different U(1) charges. The "halves" are not really associated with each other the way they are post-EWSB, although the Higgs coupling does exist. Pre SO(10)-symmetry breaking, they would all be aspects of the same thing, except that there are still 3 generations and they could be made of different mixes of generations. In other GUTs like SU(5), they may be different even before the GUT symmetry breaking.

It is my understanding that before electroweak symmetry breaking, particles had no mass.

Technically yes, but technically they have no mass even after EWSB.
The particles that "gain mass" from EWSB never behave like light propagating freely in a vacuum, which is what most people imagine when they imagine a massless particle. Post-EWSB, they're prevented from doing that by their coupling to the vacuum Higgs field. Pre-EWSB, they're preventing from doing it by couplings to the soup of other particles that are present.

Would there still be 3 generations of leptons and quarks then?

No one understands the reason for the 3 copies of the fermion fields. In most theories they are just added by fiat. There could be fermions that don't come in three copies; no one knows.
A: The question, what particles existed during the Grand unification epoch ($t=10^{-43}\,\text{sec}$; $E=10^{19}\,\text{GeV}$; $T=10^{32}\,\text K$) and before Inflation ($t=10^{-36}\,\text{sec}$; $E=10^{14}\,\text{GeV}$; $T=10^{27}\,\text K$) is an open question.
Anything outside the range of energy that we can measure and observe is and remains speculative. Sorry.
A: In one sense, all types of fundamental particles (certainly everything in the Standard model, plus probably more besides, depending on which variety of GUT you prefer) existed in the grand unification epoch, since the energy density/temperature was so high that particle/anti-particle pairs of all types could be created very rapidly.
In another sense, no fundamental particles existed since the energy density/ temperature was so high that any particles that were created would be annihilated almost immediately.
So it’s either all or none at all - take your pick.
A: It's not possible to give a definitive answer to this question yet, in part because the detailed mechanism for inflation is not even known. Inflation could be caused by a scalar particle, usually called an inflaton, but this name is largely just a placeholder.
One can still ask what the particle content of various theories predicting inflation would be, but this is also still a very open-ended question. This video lecture by Nima Arkani-Hamed gives a great sense of why there are so many possible answers to your question, specifically from 9:25 to 22:00 or so. You can get a pretty compelling model for inflation in the Standard model just by adding a bunch of scalar fields to it. Inflation comes from such a toy model having lots of false vacua, which lead to varying values for the cosmological constant and hence varying amounts of inflation. A similar idea holds in string theory. Each string theory vacuum could have not only different values of the cosmological constant, but different numbers of fundamental particles, spatial dimensions, fundamental forces and values of physical constants. Inflationary eras would then corresponding to tunneling between these vastly different vacua.
I know this answer sort of dodges your more specific questions about unification of sets of particles, but in my understanding these questions are too specific to be relevant to the pre-inflationary universe. The particle content need not even organize into the same structure that we're used to in the Standard Model.
