yeah, dp's are all very individual. Like some will attack or eat shrimp on sight, but mine totally ignore ghost shrimp. I would definitely say that they won't go well with a guppy, though. They love to chomp fins, and those flowing fins and slower-moving fish would be too much of an invitation.
I have 4 oto's in my community tank and am very fond of them. Too fond of them to risk them with my puffers. Some dp's will harass and tear up the fins of oto's, as one recent member in dwarfpuffers.com reported. I guess you just never know until you try. But, like I said, I don't even want to risk it as I'm very protective of my oto's. They're so cute and harmless that they really don't deserve any kind of harassment.