I have a Fluval 13.5 and my return pump is a Sicce syncra silent 1.0. Seems to fit just perfectly with room to spare in chamber 3. It is even a little too powerful I think for this tank, the next time I pull it out to clean it I may turn it down a bit. FYI, I also have a Cobalt MJ1200 pump in the same chamber, feeding my external media reactor. There is enough room to the side of the Sicce at the bottom that the intake filter head for the MJ1200 fits next to the Sicce in the same space.
In chamber 3, I also have 3 Apex probes, 2 Apex ATO eyes, a heater, 2 Apex DOS lines, and 2 3/8" pvc pipes to my media reactor. So, yeah, it's about the most packed chamber 3 ever
I bought the Aqamai KPS wavemaker and have been very happy with it. You can't put it on your network (it's one big downfall is that you have to connect to it's own wifi), but I find that I never mess with it now that it is setup. I'd highly recommend it for this tank.