#1: As everyone has said here, humping is mainly a dominance thing. However......I've also seen is as an attempt to get the other dog to play. I dont' think this is the case with your dog, but I have seen that be the case on other occasions. Once they start playing, the humping stops. My rattie is about 1/2 and 1/2 with his humping. He actually normally only does it to girls, but will sometimes fixate on a male. However he's not a dominant dog so I find that odd. Since he's not neutered and at some point after showing him may be bred in the future, I can't correct him for mounting. However, he does get told "off", which he listens to very well (although not always for very long........lo l)
#2: Well, I have 2 males and 2 females. I also have heard opposite sexes get along better than same sex families. Which is why when I got Gunther I got a male dane, as I already had a female Keiko. When my ratties came along, there were no issues, of course they were puppies. Both Keiko and Gunther bonded with Nigel faster. Posey is so tiny she pretty much only plays with Gunther, although when she went into heat last and her and Nigel were separated, she tried really hard to play with Gunther (he pretty much ignored her.........lo l). Both females defnitely rule the roost........a lthough Keiko has slowly been transfering the leadership position over to Gunther.
#3: I'm guessing he's just very excited to see you. One suggestion that's worked well in the past is to totally ignore the dog for a few mintues until he's calmed down. Don't even look at him. Then call him over to you and pet him w/o speaking to him. This also works for submissive pee'ers.