Generally, Reciprocity Agreements only apply to driver licensing, registration, and insurance requirements.
For instance, I have heard that CA requires a Class A CDL with doubles endorsement for towing RV doubles (two trailers), because the Class A CDL is the only license that can carry a Doubles Endorsement.
Montana has no such requirement. BUT, because of Reciprocity, I should be able to tow doubles in CA with the regular Montana license.
HOWEVER, in Montana it is legal to tow a truck/trailer/trailer combination in which both trailer have ball hitches. In CA, they say, the first trailer must be a fifth wheel hitch. I would probably get a ticket in Ca for a ball/ball double tow, even though it is legal in MT!
Another example is the tire width issue. In MT, if your tires extend beyond the fender flares an inch or two, nobody cares. HOWEVER, if you take that rig to Moab, Utah, to run the slick rock trails, you just might get a ticket. It has happened!
As to trailer braking, you must meet or exceed the minimum requirements for the state in which you are traveling. If the state (as MT does) requires brakes on all wheels of a trailer, and your trailer only has brakes of 2 out of 4 wheels, you are illegal in MT. It is extremely doubtful anybody will ever check, though.