In all of my time owning dogs as an adult (over 25 yrs, with multiple dogs), only one tried to mark at first, and he was an adult that was probably an outdoor dog before I found him. And in his defence, he was "drunk" on drugs for kennel cough, and probably had no idea where he was.
Most dogs (adults)seem to want to go out to go to the bathroom. Keeping a close eye on them the first few days, and interrupting them if they look like they are sniffing to go potty, seems to work. Interrupt them, by asking "do you need to go out?". Neutering seems to prevent most marking tendencies in my past experience.
Introducing new dogs-Pick up all toys/food/treats so there won't be anything to fight over/guard. Walking briskly together will make them bond, and feel like they are part of the family/pack. Walk your dog briskly for about 30 min, then walk the new one to get excess energy out, come home, get your other dog, and have one leash in left hand, one in right, and start walking both briskly without them stopping to sniff. Go for about 30 min. When you get home, go into a backyard (if you have one fenced), and drop the leash of the old dog, while you briskly walk the other around your dog. The old dog will probably sneak up behind for a sniff while you are walking. Keep walking. then drop the other leash, and keep moving. Try not to stand right next to your dog, and don't crouch and pat one or the other. You don't want to encourage jealousy/guarding. watch for signs that they are being polite (play bows, butt sniffing, scooting away and looking back inviting the other to chase.)
I've never had a problem introducing dogs of all ages, sizes, sexes, this way. Feed far enough apart (opposite sides of room, and you go sit near one, to make sure they don't try to go get the others' food.). Walk a couple of times a day, briskly to encourage them tobond.
