Minimum tank for one redeye (assuming you mean c. lorteti) would be 20g.
This may seem large for a three inch fish, but puffers are special and need more water volume per inch than most other fish.
As for compatability, redeyes are one of the more aggresive species so they will usually tolerate no tankmates other than a member of the opposite sex of their own species, and then only if they bond. If they don't, then they will fight to the death.
It's possible that you might succeed with a bristlenose plec without bloodshed, but it would have to be closely monitored.
All puffers have wildly varying personalities, so what one pufferkeeper can do does not mean everyone else will be able to do it too.
Examples are Pufferpack's puffer community (lucky sod) and Our Fahaka with a Bichir and two plecs.