I think you could easily keep 6 Otos with supplemental feeding / or attention to making sure they are getting fed. You might buy a few wxtra when you get them since sometimes in transitioning tanks they are sensitive. Once they get settled in and have food they usually last a long time.
But also I would wait until the tank is definitely cycled and parameters are stable for a number of weeks.
As for ferts, I would not extra dose as they can be sensitive to poor water quality. Or too much stuff in the water I would think like ferts.
You might do that in a separate tank with out fish / living critters in it.
You could supplement the Otos with fresh organic veggies. The amount you would need for them would be a low cost. Or even frozen organic squash.
There are many recipes for different bottom and algae feeders called snailo, snello, use Rapashy and or gelatin can be frozen or made each week. I like the tiny cube silicone ice cube trays. I have found one xl one at the thrift store, seen some at times, but they aren't very expensive if you look around at the discounter stores. A regular ice cube tray is too large for that if you only had a handful of Otos. Why they tiny cube trays are perfect.
I also have added rinsed (homegrown or organic) greens for them into the little cube trays especially I used for snails. You can chop up and add whatever you want.
At first You might like to sample things to them to see what their favorites are (even garlic) squash, pumpkin, melon...
Tmaier and Brookster123 have dwarf Puffers and would be good to help with which fish are compatible with them. You might consider PMing them with the thread link and asking their thoughts.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves