Moved to Nano Reefs forum...
Most of the "all-in-one" boxed tanks are decent. I have a 12gal JBJ nanocube delux and am pretty hapy with it. Most come with sponges, bioballs or ceramic rings as media for the filtration compartments. I recommend ditching all of that for the far superior LR
rubble. Also, most do not leave room for a skimmer.
It is very true that bigger tanks are easier to start with. I recommend at least a 29gal for a first SW
tank. That said, smaller tanks can certainly be kept successfully. If you do your homework, stick to a frequent and regular maintanance schedule and go slow there is no reason you should not be able to keep a smaler tank.