Hi Susan, I'm very sorry to hear about Cookie's brother, what happened?

I would recommend neutering Cookie, he shouldn't fight with the neutered male but he will mate with the female. Each animal is different, so what works for one may not work for another, they're as individual as we are. Getting Cookie neutered will just make things a whole lot easier, and it's better to err on the side of caution.
The female will be fine with the two boys, but if you do get Cookie neutered do it sooner rather than later as you'll have to leave it about a month before introducing (he can still be fertile for about a month after the snip). During that month you can swap pouches and toys etc so they can become accustomed to each others scents.
A trio to me is better than a pair in case something happens to one of them, you're not left with the one, and they're highly social animals, the more you've got to snuggle up to one another the less chance of them getting too cold.