I've had a male Beta in my 29 gallon tank for about a month now. He shares the tank with 2 mollies, 3 platys, a mystery snail, 7 molly fry and 2 platy fry. They all get along just fine.
Tony (the beta) generally likes to hang out in the corner near the filter. I have a little plastic leaf that attaches to the side of the tank that you place an inch or two below the water. Just enough for him to rest on and get some air. I recommend one of these. As long as your beta has an area in the tank he can call his own, I think he will be just fine.
He's got long flowing fins, but the other fish really don't pay much attention. I've never seen him get nipped. He was a little aggressive the first two or three days. He often had his gills out and would chase the fry a lot. He has since stopped both of these actions. So I guess if they are aggressive for the first few days that's probably normal. Also, try feeding your beta brine shrimp. Mine loves it so much more than those little beta pellets.