It's okay to add decorations as you cycle. You could practically add anything-lights, plants, rocks, etc-just NOT the fish.
Your stock sounds fine. Personally I would try it with cherry barbs first for the first few months, as they're hardier and less likely to die. Then try it with cardinals, which are sensitive but if you can find tank-bred ones they are easier to keep. Finally after three or four months add the otos, which are super-sensitive so good water quality is a must.
I'd suggest for that tank more tetras like deeper-bodied x-ray, bontesi, or serpae tetras or smaller tetras like rummies, glowlights, black neons and so on. You could also try hatchets or danios. Then you could look to fill something for the bottom of your tank. A school of cories would look nice. Finally a centerpeice fish, the crowning jewel of your tank. I'd say a nice gourami or cichlid or suitable oddball would do good.
No puffer. All are agressive IMO
and will kill fish one by one. People have success keeping otos or khuli loaches with freshwater ones but that's about it.