No problem combining 12 and 16 gage wires. Just make sure you size your fuse for the smaller wire.
With 16 ga wire I would ensure the round trip distance from battery positive terminal, out to the end device you plug in, and back to the battery negative terminal is under 10 ft then you can use a 15-20 amp fuse. That would be 5 ft wire length from the battery to the end device (Battery Tender or heated gear or whatever) one way from the battery. If the distance is longer than that, say 15-20 ft, I would only put a 12 amp fuse in the fuse holder.
I use the attached wire gage/length chart for motorcycle wiring.
I've worked with building wiring and this chart is different than the IRC/IBC electrical codes for building wiring for a reason. Building wiring is enclosed in walls and vehicle wiring is exposed to air so is cooled a lot better. I also have worked in computer, data center, and vehicle wiring systems. This last bit is for electricians who insist on the NEC for even vehicle wiring which is way more strict than needed for vehicles.
Oh, BTW, I know that the Battery Tender doesn't need that heavy of wire nor that big of a fuse. The pigtail that comes with the BT product use 18 gage wire and have a 7.5 amp fuse. I replaced my BT connection cable with a 16 gage version so I could use it to power my Slime mini air compressor which needs 15 amps.