I have three FX6s and they can be a hassle. Trying to disconnect them using the "quick connect" connections is challenging, it is very difficult to get them to release. Sometimes I have trouble getting the impeller to restart after cleaning, requiring the motor to be removed from the canister. I've never found anything impeding the impeller, eventually I can get them going again. I have contacted Fluval customer support a few times, they even swapped out one of the FX6s that was particularly problematic.
Another thing about the FX6 is that they emit a low frequency rumble sound that can resonate the aquarium cabinet. It could be annoying to some people. I've mostly gotten used to it, but would prefer it to be quieter.
At this point I am seriously considering switching to the big Eheims to replace the FX6s. Please understand, I am just sharing my experience. I am not saying the FX6s are bad filters. Plenty of people here seem to be very happy with them.
If you do want to try the FX6, it might be a good idea to start with just one and see how it goes.
I've got 2 tiger oscars, 1 Crenicichla johanna, 1 gar, 1 giant sucker fish, 2 convict cichlids, 1 jack dempsey, 1 parrot cichlid and that's it. I just gave a buddy 2 of my oscars, I think it's over stocked, but could be wrong. The convicts are the smallest and there probably 4"