Collar vs Harness small dog

Question
I just got two small mixed Chihuahua/Dachshunds from a rescue, a brother and sister, 8 months old.  They seem happy to get in their crate but, she went nuts when I put the leash on her and he just froze.  They are not very socialized to people and will not come near me unless I am sitting down.  I fear if I take them out they might slip their collars and I would not be able get them to come back. Should I use a collar or a harness?  Can you leave a harness on all the time like a collar?

Answer
Yes, you can leave a harness on the dog all the time so long as it is properly tightened. With dachshund mixes, a harness is ALWAYS recommended because putting pressure on their necks can lead to neck and back problems, because of the way they are built. I highly recommend getting them both harnesses- you can even attach tags to the harnesses just like a collar!