Yes 16 gauge is thinner than 12 gauge. OK. Here it is. The ubiquitous Wire Table. As you can see 16 gauge will be OK up to a 37 foot run with 6 ohm speakers (QS8s).

I think it will be OK to use a different gauge wire with your other speakers, but to be sure, someone more knowledgeable than I will have to confirm or deny that.

Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton