How would this experiment prove the plum pudding model?!!!!!
One observation can disprove a theory, by contradicting one of its predictions, but a single observation can't prove a theory in general. Rutherford et al. were not doing the experiment in order to see if they could support or disprove the plum pudding model. They were assuming that the plum pudding model was right, and they expected the results to be boring and interpretable within that model. They were very surprised when they actually got backscattering.
how he knew that all positive particles are stick together?!
Both the Kelvin-Thomson plum pudding model and the planetary model of the atom (Nagaoka, 1904) were already in existence by the time the Rutherford experiment was done. In both models, the positive charge of an atom consisted of a single unit. When you say "all positive particles," you're using your modern knowledge that the nucleus consists of protons that each have charge $+e$. I can think of at least two reasons why physicists in 1910 would think that the positively charged part of an atom was an indivisible unit.
(1) If it could be broken apart, then you would have atomic transmutation, which was at best a controversial idea and not something that was believed to occur commonly. (Soddy and Rutherford had discovered in 1901 that when thorium emitted an alpha, it turned into radium. But this was considered so subversive that Rutherford responded initially to Soddy with, "For Christ's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists.")
(2) The alpha particle was known to be the positively charged part of a helium atom, and in experiments alpha particles seemed to behave as single units.
why in center?
Given that the positive charge is an indivisible unit and very small, we have the planetary model, in which the atom is like a little solar system. The nucleus is much heavier than the electrons, so it would stay near the center due to conservation of momentum, just as the sun does.
