Noticed the ratfolk swarming trait doesnt specify what size the ratfolk have to be to stand in the same square without penalty... So, can they be any size? (for example, if one was medium because of a enlarge person)
Bonus Question: could the small ratfolk use the medium ratfolk as a mount & if so, what penalties to either ratfolk would there be (if any)?
It doesn't say. Expect table variation. By the rules, sure, but GMs might make a "common sense" judgement.
Probably not. Eidolons need an evolution to serve as a mount, as an example. Additionally, the larger Ratfolk is a biped.