Go for it, make sure you have proper filtration. Make sure the pleco doesnt get too big, maybe opt for a Peppermint Pleco, those get small. I currently keep a Male with some Glowfins, and cardinals have smaller fins so everything should work out. My advice is to just let the aquarium work. When I first introduced my Tetras, my Betta went into hiding and didnt eat for 4 days. After that he would nip the Tetras when I put food in the tank (I use flakes) but soon they learned to be quick with it, and the Betta gets food. I drop a pellet above him 3/4 times a day so he gets all he needs. Good luck!