A centerpiece fish is a fish that, in your tank, there is only one of. Kind of like my Dwarf Gourami in my signature. Although I think the pleco could count as a centerpiece fish.
I'm not sure on the betta though, I'll wait until someone with experience on bettas will tell you. If you take out your platies, you could add a dwarf gourami.
I think you are fine as long as you keep on the water changes. If you want to keep the platys then keep them in there. They do offer a lot of color in the tank. however, If you do keep the platys in the tank then I would not add anything else at this time.
i know most of the fish keepers on this site are very responsible, and they may disagree with me, but I think bi weekly gravel cleanings (which I assume come with a small amount of water change) will keep your tank very very clean. Not to say you are understocked, just saying, you are not in danger of being overstocked. that is, in my un-professional opinion.
Stocking appears to be fine although this depends on how much filtration you have. Your water changes and vacuuming are fine, but if your filtration is inadequate you could have problems. You should turn the tank water over 3 to 5 times an hour with filtration.