Best choice in fish EVAH! That's still a little too small for a Russels. Any of the other lions in the medium category on down would fit in there. Yes, an antennata and a mombasae would do well, but if you chose from the smaller ones, you could have more than one. Depending on the measurements, you could put a Fu Manchu, a fuzzy and a smaller radiata in there. The Fu would stay on the bottom and only the other two who be sharing the water column real estate. Maybe you could even put a little scorp in there.
As far as filtration..... I have rock. There's nothing else in there. I had a skimmer on a couple of our 14 tanks, but they honestly never looked any different/better than those without. I use an overflow like you have and that's where I stick the heater. The piece of equipment that I use that many do not, are chillers. That's because I spent the first decade in this hobby keeping seahorses, not because lions need it. I just spent too many years keeps temps rock solid, that it would stress me out to see fluctuations. Right now I have it set to 76. As Humble said, they don't have any special requirements. Keep in mind their pectoral fins are like kites and if there's too much flow, they could open those puppies up and be slammed into the glass. They can get used to anything though. When they are young it's rather comical.
If you are not putting corals in there that require too much light, keep them as low as you can so that's it's pleasing to the eye. Light that is too strong will give you algae issues. The mombasae in particular is supposed to be a deeper water fish and have big ole googly eyes. Those more than the rest should have lower light. By lower, I mean not MHs and bright SPS sustaining LEDs.... I don't mean dim. Does that make sense.
You gotta get them to eat frozen. So plan on weaning or you will drive yourself insane. We'll help you with that here.
Here's our
Lionfish Guide that may help you out.
There's a few of us on here that love our lions. You've come to the right place.