TRe, as long as the pump is 600 gph or less/w a 600 gph overflow you'll be fine. the overflow will only flow the amount of water that it is supplied with, up to its max flow rate. So what this means is, if you are using a 400 gph pump your overflow will only have a flow of 400 gph, because thats what its being supplied with. But if you try to feed a 600 gph overflow with a 900gph pump, your going to have a flood.
Hope this helps,
BM