I think that'll be good. He'll probably need 50% water changes weekly. That was the recommendation I got due to general messyness on their part. Mine spits bloodworm skins out sometimes and leaves em. Bleh!
I have Huey in a 5 Gallon Hawkeye from Walmart. The filter that came with it is a normal hang on the back filter by Aquatech (I think it's a 5-15). I had a baffle on it for Wraith and I just left it on but they really don't need a baffle. It's pretty quiet. And it can be used for cycling.
I almost forgot. Platies are omnivores with a high vegetable requirement. You could try feeding Pumpkin some pea, zucchini, lettuce, or algae pellets/flakes.