Sergeant is a morkie. He used to behave in the way you decribe on occasion when he didn't get his energy out (darting out the door, mild agression). Having you tried regular long walks? Or a "rough" play time? As for potty time, You may have to start from square one with crate training and taking him out every hour when you are home. We still have a few accidents, but they're usually my fault
. This spring we are starting off-leash training, and I hope this will prevent him from running off and getting hurt. Because it's IMPOSSIBLE to catch him once he gets going, he's the fastest bugger on the planet...
Good Luck!