So you're back to the 20g? Honestly, if you can swing it I'd start with the larger tank. My first SW tank was my 14g, which I quickly upgraded to a 30g. Then I converted my 65g and it's just silly how much easier it is to look after - the 30g is much more finicky in terms of chemistry. I can go days without topping off the 65g, but one day without topping up the 30g makes a noticable difference. I've got a little 5.5g going as well, and need to top it off morning and night to avoid big swings in salinity. The smaller the tank, the more TLC it's going to need and the less you can pack into it, but a 20g is a nice size if you can keep it to 3, maybe 4 fish and get a decent skimmer.
I use a Remora on my 30g, it's a nice little skimmer. I started with a Prizm, which is now on the 65g, and it didn't do nearly as good a job (though it pulls out some nasty skimmate on the bigger tank, I leave it alone a lot more which seems to help). I've also got a modded AquaClear 70 filter that I use as a refugium. Basically I installed a baffle and put a light over it, and use it to grow Chaetomorpha. It really helps keep NO3 and PO4 in check.
You're right when you say the rock and sand are your bio filter, though having a traditional filter doesn't hurt. I find it really helpful to have a canister (65g) and a HOB (30g) around for things like carbon, PhosGuard, LR rubble, as well as for the circulation. Regardless of other filters or live sand, you'll want around 1-2lbs/gallon of LR, depending on how dense it is. I prefer lighter, more porous rock (which not only 'filters' better, but also takes up more space than dense rock), and don't like to stuff a tank with rock, so I keep around a pound per gallon and find it's enough.
An RO unit can be essential if your tap water is unsuitable, but you may not need it. Test your tap water for ammonia (will be present if there are chloramines rather than just straight chlorine in your water), nitrates and phosphates. If you find these in significant quantities I'd find another source of water - you don't have to buy an RO unit, you could get distilled from the grocery store or WalMart. Water changes are cheap on little tanks, not so much if we're talking about the 75g.
ADD--> fire fish are neat, but make sure you have a lid - they're jumpers. Also consider one of the tiny pygmy angels (Centropyge argi or C. flavicauda, maybe a flameback but they're pricey), a clown goby, or maybe a shrimp/goby combo - a tiger pistol shrimp and a yasha hase goby would be fun in a 20g. Try to avoid fish that want lots of room to run, since it's a small tank. Clowns are a good choice, they don't do a lot of swimming about.
