I have some sort of brown synodontis cat that is six inches almost and he does fine with my JD pair. He tends to start crap with them sometimes, but appears to be able to hold his own. Mind you, they'rer similar size, only 6" or so, if they decide to breed at7-8" I will probably have to remove him. Anyway, if you can get a syno that is larger than the JDs, I'd go for it. Just watch for breeding behaviour and see how aggressive they get with it. A pair of Nics would be awesome in a 75. I once had a pair of Nics and a pair of ...well, they may have been mutts, looked like a t bar/jade eye cross. Since they were like con's and were paired they were tough as nails, but so is a pair of nics. Lots of action and movement, without any real damage. And Con like fish don't get that big, so it worked well. Male nics will eventually get big though.