Pretty sure its noted in FM:FS that most RCT Infantry Regiments could also call on a single Company of BA per Regiment.
IIRC, Its noted that this is NOT the case for March Militia RCT.
For sure not the case for Line Infantry brigades, just Front Line RCTs.
Line Infantry Brigades also don't get Jump Regiments IIRC, just Foot/Motor.
So I think when it comes to adding BA to RCTs, you get Companies w/ Each Regiment & then you get a full Battalion/Regiment added to the Infantry Brigade.
While this might make them larger than traditionally they were, I think it does nothing but show an RCT is designed to take & hold a world (for at least a short time) entirely by itself so the added infantry won't hurt at all.