Hi Martha, welcome to Simply
If you've done a reasonable amount of research into discus, you may have a sense by now that a small 2.5" discus rescued from a friend who originally got it @ a LFS, may well not have been properly cared for, be quite stressed and not in good condition, and perhaps even be a stunted specimen.
It's positive news to hear that you're doing some good clean-out of the 55 gal the discus is in, as well as upping your wc's routine - that will help you provide somewhat better water quality than before, and would likely improve the little guy's lot in life, so to speak. No need to worry about hiding places for the new guy - like many fish, if there are hiding places, the fish will hide. With time, he should become more outgoing.
Bosemani Rainbows are known to be highly active and swift-moving, and that could tend to stress the discus - however it may adapt with time if you don't move them out.
Now to the nub of your situation: If you're really interested in discus and want to get more (that you obtain from a good quality source) don't try to do that in your 55 with it's current community of fish and the small discus (who may carry pathogens you wouldn't want to see passed on to any new healthy discus).
Start out with a new larger tank, say 60 to 75 gals, equip it properly for 5-6 discus - in a minimalist environment and with a responsible wc routine, so as to make your entry into discus as successful as you can.
You can check out the forum's Sponsors section to order good quality fish on-line - go with reasonably good-sized discus only to start off with (no tank-mates), so no QT would then be needed.
That's my best advice for you to approach discus-keeping the correct way.
Hope this helps - I know this is likely not what you wanted to hear, but it's the safest approach to getting started with discus in terms of avoiding unnecessary risks and not losing expensive fish.