My experience is that geldings who have not lived with mares for some time often get excited when suddenly put with mares again, if they're used to living with mares, there tends not to be a problem.
I had a mare in with my two geldings. She was the boss and I made sure that they knew it. She came in and out first etc.
As for mares, you’ve definitely got to be of the right character to deal with one. I love them and their desire to need to think that everything was their idea, but I know friends who clash with them horribly. A good friend of mine says she wouldn’t be able to cope with Millie’s brain (granted she did say this for the first time when Millie was treating the neighbours to one of her rearing spectaculars because I dared ask her to walk past a grid that she passes weekly with no bother).