>> Some of the info I've read here appear to be older and makes refernce to loctite if I want to marry two different manufacturers?
Markins heads have a threaded 3/8" stem, upon which the clamp of choice is screwed in. If you buy a Markins head with clamp, then the factory will use a thread locker to prevent the clamp from untwisting.
If you add your own clamp to an "NQS" version (head without clamp) then you should also use thread locker for the same reason.
RRS heads use a second smaller screw, with a keyed stem, to keep the clamp from untwisting.
>> If I order the RRS BH-40LR, this would already incl the plate
As mentioned previously, either head will screw directly to the platform of any tripoid with a 3/8" threaded head stud. This includes most Gitzo tripods.