Your load as far as fish goes is on the light side. I run the same skimmer on my 700 gallon system with over 70 fish in it and the skimmer could still handle more.
That said, lower the pump speed to between 36-38 watts. You are looking for the most dense foam possible and the sweet spot for the SM250 is right in that range. Higher will result in less contact time, larger faster moving bubbles and less efficient organic removal. Lower the level in the skimmer so the line where bubbles turn to foam is about 1" above the white collar where the neck/cup threads to the body. That is about as wet as you would want to set the skimmer. If you go much higher, you will have regular overflows. If the skimmer suddenly starts producing more skimmate, shut the pump down and remove the nozzle from the pump and inspect the venturi port where the silicone air hose connects to the pump. You are looking for salt creep which blocks off airflow and results in more water flow into he skimmer. This will lead to sudden overflows. I clean my venturi ports about once a month.
As for the neck getting gunked up, that is normal and is why many of us run a quality neck cleaner. That or you should remove the collection cup once a week and clean it.
This is what I suggest as it's about the best neck cleaner money can buy and is designed for these skimmers. Mine runs once a day for 15 seconds and my neck is always clean and gunk free which not only reduces maintenance but it also keeps the skimmer performing optimally. I remove my collection cup every several months for cleaning.