I would never feel comfortable with just one on that tank. The FX5 certainly CAN handle it all, and mine has just an FX5 on it as well, but it makes me nervous.
Normally I have 2 FX5s running. I took one out to seed a new tank and it has stayed there for over a month now. The one is supporting the other tank just fine but every time I feed I get nervous. Probably just paranoid, though. My stock is a black arowana, 6 eartheaters, 3 Uaru. When I had two FX5s I had a lot more fish in here.
In general it's a good idea to have a second, just as backup, extra circulation, etc. No harm in just getting the one and adding the 2nd later if you find a deal or something.