You can't spawn new stacks via console, that can only be done with the script, which can't be changed for a game in progress.
What you can do is use the create_unit command if you want to beef up any existing Rebel stacks/settlements. The former are easier to do, since they have a fixed name, whereas sometimes Rebel captains/generals have a different internal name to the visible one.
The syntax works like this:
Code:
create_unit Thermon "hellenistic infantry hoplitai" 1 0 0 0
That creates a single unit of Hoplitai, with no experience or upgrades, in Thermon. You need to know the data names of the units, which go inside a quotation mark.