If you have neutron-absorption cross-section/neutron energy, average neutrons/fission emitted within a shake or two of the event, and density of the material, all else is number crunching. Note the world is a dirty place (impurities, multiple isotopes, phase transitions, structural disorder and voids, atmospheric neutrons from cosmic ray atom spallation, increasing temperature plus disassembly in the interval...) so there will be slop in the gears.
Look at isotope half-lives. Assembling a subcritical spherical few kg of polonium is a not insignificant cooling problem. Is polonium fissile at all? No. Also see "tickling the dragon's tail." A Manhattan project engineer sought to assemble an exponentially sub-critical mass of fissile material within neutron-reflecting blocks. As he leaned over, his bodily wealth of neutron-reflecting hydrogen and carbon set it supercritical.
"Layer-cake" cores have been tried. If you simultaneously D-T fusion (lots of 14.1 MeV neutrons) your fission critical mass decreases. However, reaction requires time versus the imploded fissile body rebounding, heating, expanding, and disassembling. An H-bomb has a thick non-fissile U-238 bomb jacket. The blast of neutrons from the fusion secondary are all well in excess of 1 Mev, the (nonpropagating) fission threshold for U-238. It all fissions anyway.