I only use .023 on thin metal like body panels. Flux core seems to burn through easily because it's so thick, .030 is the smallest.

.023 allows you to run higher power without burning holes as easily and gets flatter beads, so less grinding.

Gas IS the way to go, less spatter, cleaner welds. I use 75% argon, 25% CO2...

The easy mig from HF is pretty much junk IMO. I have one up in my attic gathering dust.
I used it to replace some rusted parts on a 914, was a PITA. Haven't tried to use it since. If I need to do any more welding I'll buy a different welder.
I like HF but I don't like this particular POS. I could do better sheet metal welding with a stick. I loaned it out to two friends who are decent welders, they both brought it back and agreed that it was a POS.
BTW, it's for sale. Any takers? ;)