Re: Thumbnail 0.2 or 2.0 question
Those are good questions :-) Here is my take, but opinions vary on this subject and many times it comes down to individuals rather than being able to generalize across a species or locale. If you just have two frogs, you will either have a pair or two of the same sex (duh...). If you have a pair, you might be able to breed them and that usually means that aggression is not much of an issue. If you have a same-sex pair or either flavor, you don't have the opposite sex in there which can be the catalyst for some of aggression you have probably read about. So, as long as you stick with 2, your odds of having a problem are quite a bit lower (in my experience).
As for which frogs to avoid, my opinion is that the primary reason to steer clear of one species/locale of Ranitomeya vs. others for a first tank is how bold they are. I have some Ranitomeya that I almost never see. That's not a problem for me because I have other tanks that I can see frogs in (including the froglets of the ones I don't see :-) so I don't worry much about it. If it was my only tank, though, and you are always looking into the tank and never seeing the frogs you bought, that might be bad for morale... So, for your first frogs, I would recommend something that is known to be bold. As a data point, I rarely see my True Nominal Fantastica or Reticulata, but I see my Vanzolini all the time. There is not really anything across Ranitomeya that makes one species/locale easier to keep than others, in my experience. Regardless of what you choose, just get your frogs from a reputable source (make sure the breeder has lots of experience and can provide you with line information).
Hope that helps,