A five gallon tank is too small for any other fish to be in the tank with a betta. It's either shrimp, a snail, or nothing. You need at least a 10 gallon for two fish. And if I were you I would just return the pleco, you'll never have a suitable tank for it unless you buy a 50 gallon tank and it's unlikely you are willing to do that. Also till then double your tank cleanings on the five gallon because of the biomass being produced. That means you should be doing 2 50% water changes and 1 100% water change a week right now, also get a lot of live plants to keep up with the ammonia. If ammonia builds up no joke the fish will both die.
And Gem it has nothing to do with the size of the animal but the biowaste it produces and at what rate. Five gallons is too small for any fish, even a betta and a neon tetra. You just can't do it unless it's heavily heavily planted and cycled and you have proven with stable readings that you are always at 0 ammonia readings. Otherwise no, it's never safe to keep two fish in a 5 gallon, 10 is the minimum for two fish.
Last edited by LadyVictorian; 11-17-2012 at 02:14 PM.