If you DO decide to go with a mix from the shelter, I personally would recommend a yorkie/chi mix (a/k/a "chorkie"). Of course I'm biased, because that's what my little boy Pixel is
He was being rehomed and their loss was definitely my gain!!
I don't ever HAVE to brush him (he doesn't mat), though I do sometimes anyway, and he is 1 1/2 years old now and hasn't had to be groomed yet. I just bathe/condition him every week or 2 and he's good to go. Very easy and such a great personality!! He also enjoys being in the pool with me being pushed around on his raft (see attached pic)
He's not nippy at all - SUPER sweet (to both people and other animals) and loves to cuddle.
He's small enough to go everywhere with me (even flown to Texas with me multiple times), but is also sturdy and strong for his size.
Of course with a mix you never know what you will get, but I've heard of other yorkie/chi mix owners who were as happy as I am with their own little ones
Good luck in whatever you end up deciding!