A powerhead or two is necessary . I was running 2 750gph Koralias in my 29g and it seemed to be the right flow for my tank, but I also had ALOT of rock in there. The fish didnt seem to mind that much. I believe the turnover rate youre looking for is 10x the water volume or maybe 15x. I always forget. lol
I was only running a hob aquaclear filter though so I wasnt getting alot of flow from that so I made up for it in powerheads.
You answered a question I hadn't even asked yet. I have a 45 gal and installed two Maxi Jet 400. I should be good as long as I have them directed correctly right?