If Mods ignored slotting rules (and potentially physics) then that would be noted, see Arsenal page 150: "Bayonet Mount: The bayonet mount allows the attachment of a bayonet (see p. 32) without using up the barrel mount, as it usually does." If they note it as a deviance from the norm for bayonets, why would deviance be the norm for other mods?
Edit for clarity
You should really double check the bayonet itself - if you did, you'd see it is an accessory, not a mod, and has "Barrel" as it's Mount entry. And while you're at it, compare the Modification listings to the Accessory listings, you should notice that Accessories have a "Mount" entry, while Modifications do not.
And again, I remind you: Mods are built into the weapon. They involve things such as swapping out parts, and that sort of thing. Mod slots are used for the measurement of how much capacity a weapon has for such modification - by default, it's 6 to everything, but the GM is free to adjust that as he wishes for smaller/larger weapons if he thinks it makes little sense for a holdout and a Panther to have the same capacity. Slots, really, would be a pointless mechanic if Mountings were required.
You should check out the pages I've listed. Had you done so you would see that I quoted the bayonet mount mod, not the accessory. And while I have listed several quotes supporting my stance, you've provided nothing but assurances that your opinion is correct. Where are you facts coming from? Show me some page numbers.
A: I checked out the bayonet mount mod. It's a mounting slot which may be used for a bayonet, not a bayonet. The bayonet is the accessory that requires the slot.
B: The rules do not contain the things that would be required for you to be right, and would be unlikely to contain a direct reference. But let me lay it out for you:
- If modifications used up mounting slots, the rules would say modifications used mounting slots.
-- The rules do not say that modifications use mounting slots. In some places, they even handle the case of having having barrel mounted accessories with modifications that would be barrel mounted if such a thing were possible
- If mountings were used as the limiter on modifications, the Slots mechanic would likely not exist
-- The Slots mechanic exists.
- If modifications required mountings, their entries would state the mountings that they use.
-- They do not state a mounting that they use.
--- This is a big hole in your argument, by the way, so, I'll ask the direct question: If modifications use mounting slots, what tells me the one a modification uses?
In other words, my case is made largely by the absence of particular things. I would not expect the modification rules to reference rules they don't interact with at all, and accordingly, they do not reference the mounting slot rules.