I suggest that unless you're planning on building a BIG cage, don't put them together. Rabbits are territorial, and if there isn't enough space, they will DEFINITELY fight, even if they get along at "playtime".
The best way to introduce them is AFTER a 30-day quarantine period. If he's at a foster's house, then there are likely other animals there as well. 30 days will give both bunnies time to adjust to the natural flora and fauna of each other.
Then, start by having their cages in the same room so they can see each other, then slowly move the cages closer until they can smell each other. Next, try a short supervised playtime. Gradually increase the length of playtime, provided you are seeing positive results.
I would NOT introduce them until you know the temperament (at the very least) of this new bunny. If he is agressive, then you should be very careful and monitor him closely.