Picking up our first pup tomorrow

Me and my girlfriend are adopting a 4.5 month old cairn terrier mix. Wondering about a few things.

We have a cat, and were wondering about the best way to introduce them.

Since the puppy is already 4.5 months old, what should we try for a bathroom schedule? Should we still plan on waking up 2 times the first few nights? Or should we go straight to about 5 hours in between bathroom breaks?

Since this puppy has lived most of its live in a kennel, do you think crate training will be a bit tougher, since im sure she's slept and eliminated in the same pen, what the best way to attack this potential issue is it does arise?

Also, she is only 5lbs currently(not do to lack of nutrition, she's just a runt). Should we walk her with a harness, or would a normal collar leash be fine.