Depends on what kind of shrimp you get. I believe Ghost Shrimp get about 2-3 inches as adults. But sense shrimp have such low bioloads you can put them in seemingly small tanks. So yeah, you can put one or two in your 2.5 gal.
Cool! Hehe, thanks so much! I will definitely look into getting one of those! :P BTW, you are lucky! You have a bunch of bettas. I want just one more HM so badly, but my parents say "Nuh-uh! You have 3!" :P They are totally right, but still... :)
It is good to have a limit though. Otherwise, your whole life will be the motto, 'just one more'. Next thing you know, you are like me who got my first Betta 6 months ago and now have 10 plus more that have passed away. :)