It's always interesting to see other people's interpretations of what is suitable as a tankmate for dwarf puffers
I'll now just add my 'bit'...I have only ever tried dwarf puffers with otocinclus and amano shrimp. Dwarf puffers as they get older and reach sexual maturity turn in to mini velociraptors. This is why you must have lots of planting in any puffer tank that will intentionally hold more than one DP.
DPs will 'taste' anything that comes in their way to see if it is edible. As a general rule of thumb, it is best (not just in my opinion, but others experience too) not to mix them with other fish. Otos are small enough and fast enough to be able to hide and get out of the way of DPs, which is why
some people have success with this pairing, me included. Otos in themselves do indeed like a mature tank with a constant supply of algae to graze on. They are delicate (not difficult) fish and don't travel well, which is why some people find them hard to keep.
If you want to try otos in your tank but want it to mature first (a good idea btw), then before adding otos to the tank, remove the puffers, change the tank around a bit, add the otos and when they are settled then put the DPs back in the tank. This way they won't notice the otos so much and will think they are in new territory and remain quite peaceful (hopefully). None of this is gospel, though, just something you can try if you want.
I wouldn't like to risk any other type of fish as a tank mate for DPs. That's not to say it can't work in some situations, just that I wouldn't try it - there's a great saying I like to remember "Never trust a Puffer"
PS In a tank the size you have, you could have more than 3 DPs. 5 Gallons a puffer (a minimum of 20L per fish) is a good rule of thumb.