The Aquaclear 200 will be fine for your 20 gallon. My 20 gallon is planted and I filter 250 gph
(1 Aquaclear 150 and 1 Penguin 100) with no problems at all. However, if you decide that it's too powerful for what you want, you can easily slow the flow rate of the filter, the directions will tell you how. As for the Fluval, it should be great for your 55 gallon, and I don't think you will need to slow the flow rate at all. The best prices I've seen for Aquaclears are at www.bigalsonline.com
and the cheapest Fluvals I've seen are at www.petsolutions.com
(I've been shopping around for Fluvals myself, as I plan to upgrade a couple tanks). When you get your new filters, it would be best to run your new filters along with your old filters for a couple of weeks, then remove the old ones. This way you won't have any problems with a mini-cycle. Sorry if someone already told you that.