i don't know where you live but...
i'm in northern california and i can tell you how we do things here. i am a volunteer w/rabbit rescue so here is what we tell people when we educate them.
after a bun is neutered, it takes 4-6 weeks for the testosterone to go away. during that time the bun can father a litter. so if you get a non-spayed female before the 6 weeks postop, you will need to keep them in separate homes. and remember that they can get pregnant thru the pen, depending on what type you have. but you would want them close enuf so they can see & smell each other. the safest thing would be to have the pens about 6 inches apart (i like to put in a board or something so that they don't rearrange the pens closer to each other. this wouldn't be an issue if you have them in cages).
also, our bunz get antibiotics for 5 days after surgery, and for boys i like to give them pain medication the first night, and for a day or 2 after that depending on how individual bunz are doing. this would require either you get the meds from the vet or get a prescription to take to a pharmacy. i don't know what the practice is where you are, but this is what we do to prevent postop infections. plus, you have to make sure that the bun is eating/drinking and thus peeing/pooping within 12-24 hours after surgery. and they do this much more quickly if they have pain meds on board. pain obviously causes stress, and stressed bunz don't eat.
keep in mind, also, that the bun needs to keep eating all the way up to surgery time. it's not like dogs, cats & people. bunnies must keep their digestive tracts moving along, they don't vomit so they have to keep eating. do not take away food & h2o from him. if you have a vet that tells you you must fast the bun before surgery, find another vet. you need a rabbit savvy vet; rabbits are totally different from dogs & cats and their medical needs are completely different also. currently, the best anesthesia is isoflurane, a tad more expensive but much safer for the bun.
i don't know what the situation is w/the girl bun that's in rescue. if she needs to be rescued right away and you can do it, that would be a wonderful thing for her.
ideally, tho, to introduce a second bun we recommend that the first bun be completely recovered from the spay or neuther (6 weeks in the case of dougal) before bringing in another bun. the process we use for matchmaking takes time, and oftentimes it takes much trial & error to see which bun dougal will bond with. we "drive" potential matches around in a cart (neutral territory and also the new situation helps them to turn to each other for comfort). if that goes well, we put them together in a big area with the family to see how it goes. if that doesn't work, we try a different bun.
however, that is ideal and where you are that may not be possible. so then you would have to introduce them slowly & gradually. i suggest you check out house rabbit society's website for tips: www.rabbit.org
best to you and dougal! if you have more questions please give a shout. 8-)