The Quietflow 10 came with my Aqueon 20 gallon kit but I'm considering upgrading to an AC 50 (or even two small filters) for my 20. Would the quietflow 10 be good enough for a 20 gallon community tank? And if so, could I take that Quietflow 10 and put it in a 5 or 6.5 gallon betta tank?
The Aqueon Quietflow 10 is rated for up to 20 gallons which really means it's too small for a 20 gallon. It's always better to over-filter a tank especially if it'll be a community tank. If you plan to put the quietflow on a 5-6.5 gallon betta tank it will likely be too powerful for a betta. Bettas don't do well in high flow and a filter rated for a 20 gallon will be a lot for a betta to handle in a 5 gallon. I'd recommend doubling your filter output for the 20 gallon or upgrade to the AC 50 (I'm assuming that's rated for a 50 gallon.)