Uuhhh... help with new cat meeting old cat, or soothe me please

Day 2 update edit: Okay, everything is smoother now. Further updates in body for everyone interested, but thanks a lot either way, all this info was pretty helpful, directly or indirectly, and just writing this all down has helped immensely with my maddening anxiety with this whole situation.

The boy got vet-ted, he has nothing serious, just a little snot for which I'll be giving him a small antibiotics treatment.

Once I find out why I can't upload pictures, I will show the kitties off, and give an update on their situation.

Thanks everyone!