Personally I think 10 months is fine if your confident he's well trained etc (we've got two 7 month pups & mum) it's hard work as you need to do lots of stuff separately with them both. Walking, recall, basic training, play time etc and then do it with them together. I've been very conscious of the fact I don't want them relying on each other & be confident happy dogs individually as well as together.
I think I'm lucky in that my two are very focused on me always (never hubby
or each other) when I'm about so training and obedience hasn't been a problem....but if you have two dogs that are more than happy to play with each other than listen to you I don't think it would be much fun & very hard work to train/ walk etc.
If your planning on getting Alex neutered at a later date or the female neutered I cant see a problem, personally I think any combination can work but its down to each individual dog, family & circumstances.
I doubt Alex will ever mother the pup (Holly the mum to our 2 doesn't even mother them)