Actually if you do CO2
you are going to want to dose the dry ferts as well. Your plants will need more nutrients when using CO2
. What type of lighting do you have?
As for Betta tank mates... that is a loaded question. Betta all have different personalities so mellow ones will often tolerate tank mates while aggressive ones won't. Cory Cats are actually the one fish that Betta's don't seem to mind, well at least most Bettas. Often Bettas won't tolerate other fish that are brightly colored or with flowly fins. So basically you have to decide if you want to risk trying to add other fish or not. Personally I'd try 3 cories and see how the Betta reacts.