A 2.5 is really a big jar - good for fry and not much else. Even then, for fry, it's good for 2-3 weeks. A pair of annulatus might be the only killie I'd consider, but even then, you'd be changing the water all the time. 10 gallon tanks are dirt cheap, probably cheaper than a pair of good fish, so why bother with the tiny tanks that don't work and ultimately kill fish?
Whilist I would go along with Gary's sentiments (he does know what he's talking about) It is the surface area more than anything that would determine more than anything what you could or could not keep, so going along the lines that you have only 2.5 gallons if it was a shallow but longer and wider container then you would be able to keep all manner of things including many Killi species within such a tank....but given that its probably a standard tank perhaps around 12x8x8 then I personally would only be using that as a short term breeder for Ps annulatus and nothing more.
Having said that it still will have its uses.....as folks have already mentioned there are some cracking Crystal Shrimp out there these days...Something I myself wants to get into at some point in the not too distant future.
atb
C