First off, I would wait on the oto until the tank has been set up for quite a while in order to attain some algae growth. They can be pretty delicate so a mature tank is a must, and algae is a major part of their diet. Ideally they should be kept in groups, but that wouldn't be possible in a tank that size.
The puffers can be difficult to sex. Females are larger and more plump, and males supposedly have wrinkles behind their eyes that the females lack. Also, there is supposed to be a difference in the spot pattern, but I can't remember exactly what the difference is.
I've actually never had live bloodworms, so I can't help you there. My puffers like live blackworms, which I've generally found to be easy to find. They usually run about $1-2 for an ounce (which is a lot of worms!). They can be kept in a plastic tub (with a lid with holes) in about a 1/4" of water in your fridge. They should keep for about three weeks, provided you change the water every other day. My puffers really love their snails, and these make up the majority of their diet. I breed mine in my betta tank. You should be able to breed them in an unheated 1 gallon tank, but a filter of some kind would definitely be beneficial. The easiest snails to breed are the common pond snails (often just called pest snails). Fish stores often have unwanted infestations of them and will gladly give you some for free.
The best way to be sure you're getting healthy fish is to see if there are any obvious signs of disease on the fish at the store (torn fins, white spots, lethargic behavior, etc). Also, look around the rest of the store and see if their other fish look healthy. There are no guarantees, though, so be prepared to buy medication if needed.
The puffers probably don't have a preference for any particular kind of plant, so your choices for plants should be limited by what sorts of plants you like and what sort of care you can provide. Different plants require different amounts of fertilization and lighting, so it might be helpful to determine how much you're willing to spend on things like that and then picking appropriate plants.
78-80 degrees is a good temperature. Unless your pH is extremely low (or high) you shouldn't really need to mess with it. Just be sure to properly acclimate the fish and you should be fine.
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