I've had a betta in my 45G community that I've been running for, eh, about a month now. I won't add anything on the nitrogen issue, but I'll share my compatibility story.
I originally had the tank stocked with zebra loaches, a banjo catfish, a goodly handful of rainbowfish, a sword, a pair of platies, and a betta. The first betta died after a couple of weeks, but I didn't notice any behavior problems. Shortly after I put the betta into quarantine I noticed my male platy was being harassed by the sword, so I put them both into the QT (10G). The betta flared at the platy at this point, but the tank was bare and the fish were stressed. The betta died and I replaced him with another. The second platy was further harassed by the sword and was placed into QT with the original; they both died but that seemed to be an issue with compatibility with the sword (and I don't know that the batch of platies I purchased from were terribly healthy) rather than anything else. The new betta hasn't had any compatibility issues once both we and the fish learned how to eat in the environment; I doubt two platies would make feeding difficult. The betta is a little shy, but he's barely over an inch in length (not including the tail.)
When I was searching around on the forums to figure out what I could put in with the betta, I never got the impression that platies would be a problem with bettas. If you want to be cautious wait to mix the fish until you'll be able to keep an eye on the fish for a couple of days. If a fish is being picked on you should be able to tell, and I (doubt) you'll have more of a problem than a chased fish.