Yeah, I agree, Danios can be a little fin-nippy and probably not suitable for guppies thats for sure. Mollies might tolerate them, but your tank is a little small to be keeping most breeds of molly (randy little buggers). To go along with Modernhamlets suggestion of a loner or fish that could keep things in line perhaps a single male Dwarf Gourami would be suitable? I dont know one way or the other so hopefully someone with experience knows if they're compatible with danios.
As far as the Ottos go, many years ago I kept them with Zebra Danios with no problems whatsoever. Ottos tend to stay out of the way of other fish and dont attract much atttention from anything else. Because they're docile and out of the way fish unless you get something really aggressive and territorial they'll be fine. My Dwarf Gourami is an un-coordinated ditz and backs into my Ottos all the time, but they dont mind
Anyway, I digress. If you do want a colorful livebearer to live with your Danios, perhaps some Platies would work too... Again, I'm not 100% sure about compatibility just tryin to throw ideas out there. Hope I at least gave you some helpful info
As far as the Ottos go, many years ago I kept them with Zebra Danios with no problems whatsoever. Ottos tend to stay out of the way of other fish and dont attract much atttention from anything else. Because they're docile and out of the way fish unless you get something really aggressive and territorial they'll be fine. My Dwarf Gourami is an un-coordinated ditz and backs into my Ottos all the time, but they dont mind
Anyway, I digress. If you do want a colorful livebearer to live with your Danios, perhaps some Platies would work too... Again, I'm not 100% sure about compatibility just tryin to throw ideas out there. Hope I at least gave you some helpful info