The lighting will be sufficient for lower light plants like Anubias nana/nana 'petite', many Crypts, Java Ferns, moss, et al.
Any root tabs will work. Everything from DIY Osmocote Plus caps to the regular root tabs you can buy at any big box store or pet shop.
While fish do remain small, keeping them in a tiny tank with very little horizontal swimming room is not necessarily the best bet for them. Doesn't mean you can't keep them - just means it's not ideal. It's always best, in my opinion, to provide the best housing for your livestock that's possible. If you can't provide an ideal setting, consider another type of livestock like shrimp.
You definitely can't keep a happy pair of Puffers in a small tank with little to no room for territory. 5.5gal minimum or 10gal, ideally.
If you can set up a larger tank for the fish you want to keep, definitely go that route.