If you can find a true isalinae then a nice school of those would be good in a 75g hex.
But imo all of the others would not be good choices for that tank.
Now the problem you will have is that there is hardly anyone that can give 100% positive id's on most of the cuda species. So finding some true isalinae's will be next to impossible.
A pretty cool alternative to a cuda would be some exodon's. A school of these are pretty nasty at feeding time. And they stay small and are very common at alot of lfs's.