Those aren't actually blennies, they are a dragonette and rely on large amounts of live copepods to survive.
Unless you have an established reef system with lots of live rock or plan to propagate or buy live copepods to feed you should not even consider one.
That said, I would not try one with the species you currently have in your 75g, even assuming you could keep one fed.
depending on his size and the seriousness of you upgrading your tank when there bigger consider a trigger or 2 and maybe a tang of some sort. IMO i would get the puffer in that 125 soon and then worry bout adding other fish.