hi there!
thoughts for you-
why not an unconventional tank size? Dont focus on the gallons so much as the dimensions.
i dunno what you have to work with but why not something that is more tall than long? or a tank that is more square than rectangular. If you could find something like two squares and use one for skimmer and then plumb it into the other then you could use the next one for return. Put baffles into it and then you would have plenty of room for all equipment. The one with skimmer could even have rubble in it and you could use that as a sort of fuge... Since you're into DIY, you can literally have anything.
Also, if you really want that 20l sump or bigger, why not cut a hole in the side of the stand? I've seen that done loads of times. you slide the sump in and then you replace the panel. Frame it out with some simple moulding, paint the whole thing to match the stand and then either use simple screws or go buy some big magnets from the HD and place them on tank and the "door" you've framed out with moulding. A lot of people will buy metal or heavy heavy plastic mesh and frame that out with moulding and paint and then put magnets on. It's a access panel that also allows heat to escape. They will sometimes put a very small clip in fan or something in to export heat. Keeps the tank much cooler and also helps with heat away from equipment.
If It were me, that is what I would do. You stand is just the standard black stand anyhow... If for some reason you busted it, which I really don't think you would, then you can also come by those super easy, atleast in my area, they are everywhere.
As far as your lights go, My mom has a very similiar light set up with hers. What she does is the MHs only blast for like 3 hours. The rest of the time it's just the T5s. Her tank is shallow and wide, so that is all she needs. She has a nice crisp bright white in there and a super blue to brighten colors. Your tank is really small. So that light is a lot. Corals will eventually love it, especially things like open brains or plate corals. Even acans would love to cook up top there. I would say have it be Blue light first (if you have separate switches) for just a few hours to wake up corals/fish, then the white light, then blast the MHs for like 2-3 hours, then go back to just t5s, then end with blue lights again. This light is not bad, it's just more than most corals are used to. I would add anything new right to the bottom and move it up every two days or so until you find that they are spreading out and enjoying the light. I have a 150 MH on a very tiny tank and ALL of my softies and brains and zoas... everything LOVE the light. The tank is only like 15 inches deep and like a foot wide. I have that light baking my tank. And everything is happy. Even my sun corals are out during the day hours, they are just in a little cave.
