if you are talking about a reef ready or drilled tank, you don't need a 2400gph pump. you can use whatever pump you want, providing it's capable of feeding the components in the sump. in other words, if you have a skimmer in the sump that has a 500 gph
pump, you'll need at least 500 gph
fed to it in order for it to work properly.
unless you have a pretty large sump, 2400 gph
may give you a micro bubble issue.
as far as the stocking of a 180, i wouldn't put 5 tangs in one. 3 would be the limit in my opinion. i'm not talking bio load...just crowding.