The tank size is definitely fine for a single dwarf puffer and the buffering provided by the few extra gallons will help you keep the parameters good. Sounds like a nice set up.
Some of the slower growing plants require very little trimming at all and do very well for beginners, especially java ferns -- the daughter plants look pretty as they grow and once they float free, you can just pull them out if you don't need them. You just want to make sure you don't plant them as you probably already know -- tie them with thread to a rock or driftwood so the rhizome stays exposed. Java moss tied to wood also looks great and won't take over the tank any time soon. Crypts and anubias are slower growing and do well in the beginner tank. Like the java ferns, don't plant your anubias.
Some of the easier but faster growing plants, placed in the back of the cubes can create a dramatic backdrop effect when allowed to keep growing so the tops float on the surface (thinking Vals especially, which I love for this look, and elodea, too) and if you get tired of that look, they do fine if you snip them back when doing water changes.
Ghost shrimp are frequently kept as dp tankmates, but it is a gamble. I haven't had a problem with them, personally. I wouldn't try other shrimp unless you don't mind offering expensive feeders. Yep, snails will be gone.
On otos, though I know you didn't ask about other fish, they go well with dp's, but I wouldn't add one or two to a small tank personally because they are actually schooling fish and do best in larger groups.