The formula is simple.
4man sections - 1 weapon slot, can have medkit or pyro
5man sectioms - 0 weapon slots cannot upgrade.
My fav change however would be to make sections default 5men with 1 slot and pyro/medkit and the bolster upgrade for 6 men and no slots or upgrades.
Edit: obviously nerfing 5men sections to not have slots would mean an accompanying buff to stock sections to compensate and make the 5man upgrade attractive. Same with the 6man version.
I'm not a fan of 6-man Sections, and I would rather not make 5 men the standard squad size. I like the design philosophy of the Brits as a smaller, more well-trained force within the Allies, and I think giving 4-man Sections advantages over 5-man Sections would speak to this, as well as add a bit of versatility to the faction.
I also think that, in your proposal there, I'd rather 5-man Sections get an extra weapon slot as opposed to having theirs removed.