Sounds like a good amount of flow. What do you plan on having in the tank?
I used to run 2 850s and a 250 in my 40b. I now have an MP40 going across the front with reef crest(random flow) and an 850 constant movement across the back. I have a 250 on back wall pushing water to the front wall.
If it were me, I would get an mp10 on the 20g. They are great, easy to use, easy to clean, no power cords in the water, and will allow for future expansions!