If they are both neutered, they'll get along better. We've had eight neutered toms, mostly rescue cats of various ages, in various groups and they've all got along OK. Some better than others - Felix loved everyone and everyone loved Felix; Penfold is grumpy but likes Bertie best; Diesel seems to like everyone. Tom, our new boy, is quite timid still.
As long as there is no competition for food, it's a lot easier. Ours spat a bit from time to time but it's mostly handbags at dawn, rather than real tomcat fighting.
But snip, snip, vaccinations and let him go. CP also test cats for FIV. I don't know what they do if they test positive but I'd do this before the snip or vaccinations, just in case euthanasing is the route for FIV cats.