Is there a handy list somewhere showing how many tons HQ capacity each commander rank can handle maximum?
There is no max capacity for rank, but the game assumes a rank based on unit size and/or HQ capacity.
Unit size
<=5000 Major
<=10000 LtColonel
<=20000 Colonel
>20000 Brigadier
HQ Size
>75000 Mj. General
>250000 Lt. General
>1000000 General
So if you create a unit of 7500t size, Aurora will assume that it should have a Lieutenant-Colonel commanding it.
If you create a unit with HQ capacity above 75,000t then Aurora assumes it should have a Major-General commanding it.
You can of course change these things as you see fit.