Would like to add more power to a ZW-PWC

Just go with the 180s. There is no such thing as too much power, you may need it down the road.

You can add 1 at a time, just remove 1 jumper on the back of the ZW-C

You can mix 135W and 180W bricks on that one, but I have to go with Doug, the PH180 is a vastly superior transformer and I'd just expand with those.

Question, are you using Legacy on your layout?  Some ZW-C's had issues where they acted like they got the "Halt" command when used in a Legacy environment.  If you don't have enough power, what would happen is just the transformer involved would pop and you'd have to reset it, not the ZW-C.  If you are losing the whole ZW, it may be a Legacy issue.