I'll try and help you some... from what I've learned from our three.

Barking at night: I'd say he's lonely and wants in to be with his family, assuming that the other two dogs stay inside. I'm really not too sure how to deal with him barking though, other than bringing him inside at night. Our three are definitely house dogs, and sleep inside in our master bedroom. Zeke has his own bed, and Bunny and Eli sleep in an over-sized chair and snuggle together. I know some people do crate their dogs at night as well.

Leash: He's probably just not used to a leash. If he's rarely experienced using one, then its understandable. Eli did a similar thing when we first got him. I use a Y coupler when walking Bunny and Eli. It helps to keep them orderly and also I think it helped Eli learn how to properly walk with a leash. You might look into one, and this way he'll not only learn what to do from you, but also with the help of your other dogs. Also, bring along some tasty treats to help lure him into following you. Maybe some cooked chicken or something else that's just extra tasty to really draw him in.

Reactions to other dogs: I think this sounds like classic pack behavior. Our three do it too. Eli is the most relaxed dog, goes to the dog park alone with no problems. But at home, he is guarding his family. Zeke used to be friendly with other dogs when J first got him, but since Bunny joined the family (and Eli later on), he gets along with her and no one else.

Playing outside: I would suggest using a specific word to instruct them when its playtime and when its potty time. We use Outside and Potty. If its time to eat, they go out before and after. I open the door and say Go Potty. If its just in the middle of the day or whenever and they need some outside time, I say Lets Go Outside. Also, again, rewards after they've done their business is an option.

Intro to other dogs: Using muzzles works well when first introducing dogs to each other when you dont know how they'll act. Also, introducing them on neutral ground helps. When we brought Eli home, we introduced him to Bunny first. We muzzled both, I had Eli on one leash, J had Bunny on another, and she met me across the street, in a common grassy area that's neutral territory. We let them sniff each other and walked around some until they were comfortable. Then B went back in and out came Zeke and we did it all over again. Then we brought them all in the house and kept Bunny and Zeke muzzled until we were all comfortable and adjusted. Dogs are all different, and will take different amounts of time to get used to others.

Hope all that helps! =)