With 1 bulkhead, you have to do some sort of 'Durso' style drain. They work fine, but can be noisy. 2 bulkheads will allow you to do a 'Herbie' type drain, which is quieter than a durso, but take a bit of tweaking from time to time. 3 holes will let you do a 'bananimal' system, which is as quiet as a herby, with redundancy.
If I were going to be drilling my own tank today, I'd probably go with a Glass Holes kit. Nice design, everything you need, reasonable pricing...
I'm converting my Durso to a Herbie in my reef ready cube this weekend... don't like the noise.
As for the LFS that said you're an idiot... Spend your money somewhere else. There are dozens of reasons why you would want a sump under a 20g tank. I'm quite sure, if you go to the nano sections of this forum, that you'll find folks with much smaller tanks, and sumps. If nothing else, it allows you to use in-sump skimmers, get the equipment out of your display tank, use an ATO to keep salinity and water level stable, add macro algae for nutrient reduction, the list could go for pages.
1 fish? Well... if you want a puffer or a lionfish, then yeah, you should probably stay with 1 fish. I once had a 20g tank (10g sump) with over a dozen seahorses in it. Had a lot of fun with that one. Keep with smaller fish, good biologic filtration, and you should be able to support a nice variety.