I tend to keep 3-4 fish in a 29 gallon (+15 sump). But, it is all about the type of fish, how much waste they produce, and how good your filtration is. I have 2x clowns, 1x midas blenny, 1x 6-line wrasse.
T5 lights would also be great. You'd be ok with a 4xT5 fixture. That would support many softy corals while keeping the bulb, fixture, and electric budget down. But in this hobby we like to think 'more is better'. 6x bulbs would be great and is very flexible. I wouldn't do 8x bulbs unless you think you really need the extra watts. Some people scream that you have to go big. But ultimately you'll end up paying for bulbs and electricity that you may not need for the critters you keep.
Myself. I use a single 1x175w halide without any T5's on my 29gal. But I keep anemones, and 8xT5HO's are far bulkier than a single hanging halide pendant. But, the elec bill certainly did go up
My new tank... soon to be put in... and it's related electric bill is going to shock the wife. I don't think she realizes it yet... and I fear telling her LOL. 3x 250w halides, 4x T5's, with LED moonlights.
A GREAT plan is to add a sump w/skimmer and maybe a reactor for carbon/gfo mix. The heater can go down there too. Best thing I did for my tank. You'll find it opens up the display, keeps the water parameters far more stable, and you now have extra room for more live rock (and chaeto). I like to cram live rock in the sump, and try to reduce the amount in the display to open things up for the fish. Plus allows for some leeway with how you do your rockscape.
A 20 gallon long tank with a few baffles siliconed in would be an excellent size I'd think. But anything is better than nothing. So if you can only fit a 15g or a 10g. Then that is fine too. Also.. go with a drilled overflow... the hang on back ones are convenient to start, but I hate mine. Always an eye on the tank for fear of a flood.