He is most likely going to be programmed to stay in the cave. Yes, the pleco could share is space. One method that works (sometimes) with getting timid fish to come out is adding dither fish. Doesn't always work, but when the dither fish (usually giant danios, tinfoil barbs, very fast fish that school) are out schooling it is a sign that it is ok to come out of hiding. I have this problem with my breeding paid of JDs who hide a lot, I would put dither fish in there but when they breed I would have to sell them (don't have space elsewhere).
Another reason fish sometimes hide is if another fish in the tank has claimed dominance or territory. They continually run the other fish around the tank and out of the dominant fishes territory causing the less dominant to hide. To try to fix this, usually rearranging can work. Sometimes only for a little while, but they have to set up new territories when their tank isn't the same. Or you could get rid of the bully haha.
In regards to the fish you are keeping.... What they said about the Oscar is 100%, they can get 15" or so, and require much space. A pleco can usually share space with anything, just keep in mind some species can reach close to 30".
Let us know what you decide to do (if anything) and give us some pictures!
