I suspect that each of those four vendors will tell you to stick with ONE BRAND and not mix them, to ensure there are no compatibility problems. Might go with 3M as they probably have the largest line of fiberglass refinishing products. And they'll probably tell you to use a prep solvent or prep wash at various points as well.
Again, any one of them should be able to tell you how to do the whole job from start to finish with their own products--best to confirm it with the maker you choose. Often "gallon" is the most economical size since it is cheaper than two quarts of the same material. And terry cloth towels (always and only white) beat the heck out of every kind of rag for absorbing things from the work. Wash dry reuse, buy 'em on sale.
With rubbing compounds it pays to start with a LESS aggressive compound than you think you will need. Try it, go more aggressive if you need to--but start with the least aggressive one that you think will work. Even if that means buying just a pint of this or that to try them out first.