Looks good so far.
As for the question about biofilm, it is virtually all bacteria. Heterotrophic bacteria colonize water almost instantly, you can feel the slimy film in a new tank within a day or two. It contains a number of different species, some of which will be the same as the ones in the filters, many of which are not. Water also has other things in it, fungal spores, algal spores, which are in the air all over the place and grow whenever they find a suitable place to do so. But algae growing on the tank walls isn't biofilm, at least not as the term is usually used regarding aquariums.
The six month figure is simply one that is often quoted. Tanks will mature at different rates depending what you keep in them. Otos do eat biofilm, but based on what I have observed of the ones I've kept, if they don't have a fair supply of real alage to eat, they tend not to last very long. They have a natural life span of many years.. I'm not sure what the maximum is but they are not short life spans.. similar to many cories I think.
I would expect that Otos get their gut flora from their environment, and what they eat. It's probable they can get at least some new ones if they have fresh growing algae to consume. The problem arises if they starve before they can grow enough new gut flora to be able to digest enough food to carry on.
drip acclimating for the first time now so we shall see how everything goes 
