You know you can move the queue from one TC to another, right? If you have 6 days in three TC20s, and you want to run TC19, pick the one which is closest to putting out a hero, and remove one (or two if you have the houses to do it) from its queue, and then add that one to one of your others. Repeat until it only has the one it is currently half-way done with. It will be free as soon as that one finishes.
You can also move recruits from TC20 to TC11, pulling food out of the queue… or from TC11 to TC20, storing food away in the queue.
For this reason, I expect to have one TC20 and one TC11 both loaded up for as long as I play the game. My other two TCs usually have shorter queues and I use them in different modes.
The best sure-fire way to use 6 extra Gunnars: Train one Gunnar with 3x Gunnar (75%) + 6x 2* blue (24%) + 1x 1* blue (2%) = 101% chance of special increase = guaranteed increase each time you do it… Or just feed them one at a time and hope for the best.