I think a second cat is a great idea
As to whether it should be a maine coon or a rescue kitty - well I'll be a bit controversial and say that while I think you should consider both options carefully, you should also listen to your heart. If you really, really want another coonie you may always regret getting a rescue kitty because you felt you should when your heart was pulling you in another direction. Breeds do have specific characteristics and you can predict what they will be like as adults much easier. As they're large cats, I think you need to make sure any new cat is or will grow to a good size too otherwise there's potential for the newbie to be bullied, esp if the newbie is less dominant, or hurt during rough play. You might find that when you start looking at rescues one mayl have a kitten or young cat that looks a bit like a maine coone and which has the purrsonality you're after, of course. And whatever you get, you will love him/her to bits.
I'd also keep in touch with the maine coon rescue people in case a rescue that would suit comes up. Mainstream rescues also may get a maine coon or maine coon cross in so it would be worth speaking to a few local rescues to let them know that you'd be interested if they did get one.
If Leo is a dominant cat then I agree that you are best off getting a slightly less dominant cat but you need to be careful you don't get one that's too timid and easily bullied. I ended up with 2 dominant cats and we have fireworks at times, but mostly it works because Jaffa is bigger than Mosi so has managed to hold on to his place as the boss.
Age wise - I don't think it necessarily has to be a kitten - a young cat would do as well, imo. And if the cat is a little bit older, he/she will have the purrsonality a bit more developed and it may be easier to predict how he will get on with Leo. If you go down the rescue route, I think an older kitten or young cat would be better than a young kitten as you will then have a better idea of the cat's adult purrsonality.
I've only had male cats. I don't think you will have any more/fewer problems with introductions than with a female so I'd just go for which ever you prefer.