I would put the newcomer in the bathroom for at least a couple of days,
while you bring blankets in and out to get all cats get used to each
others smells. Also a couple of Feliway diffusers will be helpful.
Perhaps some Rescue Remedy in the water.

Victor, who apparently likes to drug cats... :)

Or if your bedroom has a window, I'd use that first so that long periods
would be more interesting. Then to switch them around, put your boys in the
bathroom and let the other cat out for bits of time. If you know someone
handy you could put a screen door on the bedroom. I just think you have to
take it slow and see. It's so personality dependent. It took two days or
less in college with my Amigo and my roommates cat to become best friends,
but it took me months to intigrate Pearl. Bless you for doing this and a pox
on those mean people for sending a senior cat to the shelter :(

You've got good advice so far. I don't have anything to add except to
say that it can definitely be done; how long it takes depends on the
individual cats. I've taken in three senior cats and successfully
integrated them with the rest of my bunch. The oldest was 17 at the time
he joined us. I can't resist older cats. Their on-the-job experience is
priceless :)

Good luck :) Its not easy introducing new cats.
This is a good link about cat intros , it's  long but worth reading
http://messybeast.com/first-impressions.htm