Hi Bapt,
I could see an infantry only option as a possibility. Here are my immediate thoughts...

Infantry only could be an "event" you select at the beginning. This could just be an optional button you select, similar to selecting the "end-game Objective".

I use "Vars" to control most of the spawning.
I would have to go through the script and create a "switch" with a "case" and a "default" every time the script creates a variable value for vehicles and cannons.
The "case" would say: if "infantry only", create values for infantry Vars.
The "default" would create values for infantry, vehicles and cannons.

For the player it would be as simple as preventing the vehicle reinforcement button from appearing. This would be a similar "switch" in the "main menu" trigger.

Possibly restrict how much MP the player gets at the start too?