I think in this situation its a big toss. No one can really say if the addition of another MI is going to go bad. I will say that the fish that remained at the LFS is already compromised (you said he was beat up). You might ask why that other fish is beat up? Was it previously with your Idol, for instance, and they fought?? Questions for your LFS before making an informed decision.
I advocate strongly for quarantining all new fish, but it seems like you added the new Idol into your display without quarantine? Especially with a stress harassed fish, I would not advocate for adding another to my display directly.
I don't know at all your type tank nor your exp. level in the hobby, but MI, as we all seem to know, are not fish that usually survive in hands of most hobby fishkeepers. You have the fish now, so you will need to make decisions about how best to proceed. If it were me, in your situation, and I decided to get the 2nd fish (after getting more info form LFS) I'd QT both fish together to see if they are compatible, and allow both fish to experience isolation and stress recovery and possible disease treatment before putting them in the display.
Responses here by members, I think, were based on a conscientious voice advocating against keeping MI in the hobby. Doing so would represent our advocacy for this fish and would discourage fish sellers from harvesting these fish (that have low survival rate in captivity). Generally, in any fish forum, advocacy for proper care of the animals we keep is a common theme. Hobbyist everywhere advocate strongly against harvesting, selling, and keeping animals with low survival rates in captivity.
Hopefully all will go well in this situation, and I'd be interested in hearing updates. Good luck!
