I personally wouldn't wait, especially if you don't want 'studdy' behaviour & not confident about how to deal with it. 10 months is already getting too late IMO. You might find the testicle is just twisted & stuck higher & can't be felt.
I had a young boy I thought only had one. Luckily I had a very good vet who said he was happy to do the op in the paddock & not charge extra if it wasn't descended. But he found the testicle was there, just couldn't be felt from outside. Said that was not very uncommon.
Re dealing with nipping & aggressive behaviour, I don't personally adhere to the 'dominate them' attitude, but I think it's also more risky with a hormonal horse, as 'dominance games' are what they play with eachother & I would not wish to enter into that game & be seen as a challenger.
I prefer to use positive reinforcement(reward) training to teach the horse the Right behaviour instead of getting focussed on the Wrong behaviour. After all, if you teach them they get a treat or such for having 'manners' such as staying 2' away from you at all times, being 'polite', then they stop bothering trying to 'mug' you.