Required GPH is determined by the amount of flow you want in the tank, not the size of the tank. Usually determined by the livestock (i.e. SPS, softies, LPS, FO).
Inline,
if you're using it as a closed loop you're all good although probably better served by two powerheads as they're take up less wattage. If it's for a return you may want to throttle it back a bit. I have an Eheim 1262 on my SPS 58 oceanic and I keep it throttled back. At full power the circulation is great but it pushes way too much volume through my sump. When throttled back I get no microbubbles and my skimmer seems to be more efficient (although that could be in my head)