My Rescue Dachshund is marking everything!!

I just adopted a 4 year old pure-breed dachshund from the Humane Society about a month ago. He is the most loving thing in the world and is the epitome of a lap dog- however i'm having a couple of issues with our training. First, the marking. I have a female rescue dachshund already in the house and they are getting along great, so I don't think it has to do w/ her (they are both fixed). He doesn't do it everyday, which is why i'm having a hard time figuring out what the cause it. He went once (quite a large amount) on his kennel, then this morning he lifted his legs all over my clothes which were hanging on the doorknob of our laundry room door. He knows its wrong because he scampers away. I don't punish him but rather if I catch him, i'll clap or startle him out of it and then take him outside, but the damage is done (i use Nature's Miracle immediately on the spot- so he hasn't marked the same spots more than once- its been different places every time).

The other thing is other dogs. He really is rather uninterested in other dogs (not like my first dog who will choke herself trying to get to another dog during our walks). When we DO have to interact with another dog, he seems fine (sniffing, no growing, etc.) and then he'll turn and snap at the dog. I've had dogs my whole life and know what signs to look for but its like he goes from 0 to 60 faster than any other dog i've ever seen. I'm not sure what is going on or what I can do to help.

Rescuing is always frustrating because I have no idea what his history/backstory is so i'm not sure how best to approach his training. Maybe a few accidents in the house is to be expected while he's still getting used to us? I just don't want to allow bad habits to form. Any advice is much appreciated!