I'd agree, lots of fish for a 55. Your stand looks about the same size as my 55 FW
, not much room under there for a sump unfortunately, and you'd probably have to take the tank off to make it work. If you have the room, you can always put a sump adjacent to the setup and enclose it separately. You could make a nice stand enclosing a 30 long and hide everything nicely. I started the ATS thread so if you want to check that out as an option (it's just ONE example, keep in mind), DIY is pretty easy but needs some forethought. It would help the nitrate problem once established. Then you can actually feed as much as you want, any excess uneaten food actually feeds the filter (algae). Some soft corals actually prefer a little excess nutrients (nitrate, phosphate) so it might not be that bad.
Also, just FYI that's not a Sergeant Major, it's a four-stripe damsel or Black-tailed Dascyllus.
Overall, your tank looks really nice though! Hard to tell that you're having any problems.