Lump - and a cold?

[Lolo76]

I've had my breeder mice for a few weeks now (2 females & a male), and recently noticed two possible health issues. First, the boy has a lump on his back, about the size of a small marble... I suspect it's an abscess, rather than a tumor, since it popped up immediately & seems to be getting smaller. Could this be from getting bitten/scratched by one of the girls? And how should I treat it? I read something about squeezing them, and then applying neosporin on the wound - but I'm afraid of hurting or misdiagnosing him.

Then a few days ago I noticed they were sneezing, about every 3-5 seconds or so. I'm assuming that's a cold/RI, and was hoping some tetracycline in their water bottle will do the trick... I have a leftover package from my old pet rat. No offense to the mice, but I'm not about to spend $$$ on vet visits & fancy antibiotics, especially considering their purpose. Any thoughts or advice on these issues? Am I on the right track? TIA!

P.S. Aside from these two things, my mice are doing quite well... I think one female is pregnant, and I found the new girl getting "cozy" with the boy tonight.