That sounds about right, but I'm more familiar with the old size designations for the AC
's....I have a 70 (old 300) on my 55, and what used to be called a 200 on my 10g planted, which is a little strong, but I turned it down and it seems about right. So if the 20 is the old 150 that should be perfect for the 10g planted. For the nano reef you could probably go one size up as that way you'll get more flow.
I love the Aquaclears....great filters and I have yet to kill one. I just wish they didn't change the numbers as I always get confused when I go to buy more media!