We're still talking about koralia 425's right? Anything more could give you trouble, it didnt for me, but I like alot of flow. Two 425's will be enough, 2 550's would be nice too.
I use 2 750's in my 55g as well. i was even using a 750 in my 29g biocube with absolutely no ill effects, but that probably has to do with all the live rock I have in there and how it's placed.
every tank is different though. If you ahve alot of rock I thinkmore flow is needed. If you have very delicate fish though that should be considered as well. My mandarin doesnt mind the 425 in the cube but I havent tried her with the 750 yet. But I will be.
I think the water turnover youre looking for is at least 10x the water volume for fish only and 30x for reefs. So 1000gph total with the filter included is pretty close.
You could also buy just one and scope it out. See what it does for your tank.
You have a 30" long tank correct?