Hi,
It’s great that you’re doing research before getting more bunnies!
As you know it’s important to spay female bunnies for health reasons, but it’s also essential to a good bond that all bunnies are neutered.
When you buy a baby rabbit, there’s no guarantee that they’ll get on. I did the same thing, and bought a baby as a friend for my adult, so I’m not judging at all. That said, it was a tough 4 months keeping them separated, and at one point I really did think they were unbondable. You do need to be prepared to keep them totally apart for quite a while, and you need to know that they might never bond. Unfortunately, this just happened to one of members and she had to rehome the second bunny. Of course, mine are now bonded and happy, so its not all doom and gloom!
To answer your questions one by one:
Which gender would my female most likely prefer?
Generally, neutered male/female pairings are easier, but lots of people have happy same gender pairs. It’s more about personality than gender, but generally, a bond between a neutered male and a spayed female is easier.
I’m open to two babies, even, so would it be safe to take two litter mates at the same time?
It would be fine to take two litter mates, but you would need to separate them between 10-12 weeks before they get hormonal. Then, once they were neutered you would have to bond them either together or with your existing female. So this would mean at one point, you would have 3 rabbits housed sepatately.
Would any of them show aggression to each other or to my older bunny?
It’s very likely that as they sexually mature, two littermates will start fighting. There’s no way of knowing how they’ll react to the older bunny, but it’s likely that they won’t get on immediately. Either way, you will need to go through the bonding process.
Or would my older bunny get more irritable with two little ones as opposed to one?
It is much harder to bond a trio than a pair, and often one bunny ends up being left out. That said, if you have a look around this forum, there’s a few people with happy trios so it definitely can be done, but it is more work.
SO… to sum up. Either way you’ll need to keep baby/babies separated from existing bunny until they’ll all neutered. After giving them a month to recover from the neuter, you can start prebonding them for a month (swapping them all between cages). Then you can start the actual bonding process. There’s lots of threads on here about the bonding process, but we can help you out nearer the time too.