I havent weighed my cats for a while now (it was done at the vets). I dont have any weighing scales in the house, but Mr Bengal feels very very heavy. At a guess I would say he is about 6-7kg and is a fat cat. The pics above of when he was younger show his face to be quite lean...this is no longer the case.
Reflex is probably about 30% lighter and small/slim/lean in comparison to his friend. He is also stronger than Mr Bengal.
With regards to introducing another cat into the house: I actually think that my 2 would be happy for a 3rd cat to come in, though this 3rd cat would have to be of an athletic nature, like the 2 I already have, as Reflex in particular, would harrass the new arrival, non stop. If my current 2 were more laid back/slow/old, then I would look for the new arrival to have a similar personality.