Pump to tank size

[bear27]

I am putting together a system which consists of a 50g main tank and a 20g tank for a sump with duel returns. I was led to believe that the more flow the better so I got a mag 18 for a return pump, but From reading this post I'm starting to think that I have to much flow for the size of my system. Right know I'm still researching my sump design so I have no baffles yet, Do you guys think that this is to much flow for the tank and if so can you pinch back the pump to reduce the flow