I am installing some landscape lighting. The kit came with 14 AWG wire. I have 10 lights at 11 watts apiece and a 150 Watt power pack. I need a few extra feet of wire and have some 16 AWG laying around.
Is it OK to mix the 2 sizes?
Generally its not OK. You don't say whether the power pack is 12 or 24 VAC.
AWG 14 wire is rated for 15A
AWG 16 wire is rated for 8A
So, if you are using a 12 V power pack, 16ga is too small.
It would be technically OK, current-wise, for 120VAC, but I don't know of any 16ga wire rated for distribution outdoors.