Hi. I foster for a rescue. Do you have any other birds already? If so, you need to worry about these birds carrying disease.
General considerations: Can the rescue provide proper cages, food, and other supplies for you? What's the situation with vet care? What's the procedure with prospective adopters? What is role in education and/or evaluation of the adopters?
In general, it can be more difficult to work with a large number of birds at once, though the idea of observing them and figuring out who gets along with whom is a great idea. We only adopt out same gender pairs* are therefore careful to prevent mixed gender bonding while they are in foster. I have budgies of both genders here, but it's something I watch carefully and I never house mixed genders together. (They do get out of cage time together, though there were two who I didn't let out at the same time).
If he thinks they'll be adopted within a month, you probably won't have time to do much more than get to know them better and perhaps choose one to go with the guy you already know you want. Does he encourage adoption in pairs? If so, you might want to house them that way once you figure out which pairs seem to make sense.
Let me know if you have any questions about anything. I find fostering very rewarding and a lot of fun. You do have to ask yourself if you'll actually be able to let them go.
[* or a single bird if somebody will be home enough. Or a single bird when they already have one of the same gender and they are looking for a buddy for it.]