An active cat is more likely to accept a new kitten. A quieter cat might prefer a mature adult. The most potentially volatile combination seems to be two uncastrated, mature, male cats.
Territory is important.
Reduce tension by making sure each has enough personal space and possessions to meet his needs for territory. Have at least one litter box on each floor of your home. Consider placing litter boxes in various locations to avoid the exclusion of one cat from another cat s territory. Keep scratching posts and beds in several locations to accommodate all the cats in your household. Multiple bowls of clean, fresh water is also a good idea.