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!
Depends on what you’re trying to keep in there. I personally don’t use them anymore because probably twelve out of the twenty or so I’ve used have ended up shocking me.