Better to beg forgiveness then to ask permission.
Honestly, I just want to point out this is how a lot of bunnies end up homeless. Not that I think Rue or her husband would abandon a bunny, ever, their devotion to Gus proves this, but it kind of worries me that people give out this advice at all, TBH.
For example, Fidget and Midget, the two bunnies I just had to rehome when they were surrundered, were with that family under the same sort of ideal. Daughter went out and bought two rabbits, only to give them up not a month later because Mom was so upset about it. They were both there to pick them up, but that didn't mean Mom was happy about it and I don't think getting something that will leave a negative first impression like that is a good idea, it's not really fair to anybody. I honestly think that when making a decision as important as another family member, it shouldn't be 'Oh, lookie what I got today!'
Rue, honestly, if you're going to make this decision I think it would be better to take it slow and find a bun you all like, including Gus. If you happen to get the wrong bun and 'Surprise, we have a bunny!' and perhaps it's just not the right fit, it wouldn't be a great situation for anyone. I've just seen too many disastorous results happening from the surprise method and I'm rather shocked that so many people on here seem gung-ho for it.
The fostering thing would be different, because then you guys could get a feel for the bun before really commiting and even if you decide against that bun in particular, you're still doing a good thing for that rabbit until it gets adopted and then you try for one that's a better fit. I still think going to the shelter and looking at buns there, taking the time to play with them and meet them, is the best way to go.
Just my two cents.
