My understanding is that most of the initial die off among oto's is from the poor handling/treatment they receive while transported (at least this is apparently dominating belief now). When they are captured and transported, they are not given the food that they need (algae and diatoms) since they are usually kept in clean water. This causes them to lose symbiotic bacteria/microbes in the gut which aid in digestion. If these are lost, then even when they are given food, they are unable to make use of it. (I don't know any of this personally, just from a lot of reading/conversation)
If your oto has been living happily for several months, then I would think that it should be fine to transfer it to another tank, because you have now seen that it is healthy and over the period of worry. And as Mofo said, www.otocinclus.com
is a great place to start for info.
 As long as your 30 gal
tank has algae your oto should be fine in there, but I often see oto's apparently eating off of the leaves of live plants. I don't know about the compatability with the other fish.