I don't know what our build rules say, I only know so much certain weapons have a different wheight than in CBT.
One I know of is the MBL.
Also, instead of a criticals we use a podsystem, as has been mentioned about a bazillion times before!

This does apply to all weapons, which means thath the Cougar might not have enough space to carry the big missilelaunchers, like lrm20, atm12 or CDSSRM6 in it's sidetorso, and the biggest one it can fit is the CDSSRM4.
Most importantly tho, it has also been stated numerous times that KingLeer went, or is going over all, I repeat ALL the variants in the build which of course include the Cougar D, if he finds that variant to be underweight by our buildrules, something is going to be done about it, within our buildrules fo course. At the very least you're going to get two additional tonns of ammo.