That is a dilemma. . .
One the one hand, schooling fish like to be in larger groups, and you would probably be ok right now with 3 more of them.
But, your pleco will grow to be too big (7 inches) for a 10 gallon tank even if he was all by himself. . .
So, if you have plans for a larger tank in the near future, then I'd say go ahead and get some more Cardinals.
I know basically nothing of Bettas, so if there is a Betta-related reason why you should not have Cardinals then someone else will have to bring that up.