Surely the best option for 16 players is to have four foursomes which is
covered here and elsewhere as the social square. Or are you considering 4 threesomes + 1 foursome?
With 18 players I think you may need as many as 8 rounds (of 3 foursomes + 2 threesomes) before it is possible to have a schedule where everyone plays at least once with everyone else, there will be many pairs that play twice. Can you manage that many rounds? But if you wanted 5 rounds then see Ed Pegg's page where he discusses
How long can 20 golfers play in foursomes?, simply delete a pair of players such as A & G, who never occur together, to derive an 18 player schedule. Note that as the 20 player, 6 round foursome schedule is known not to exist, then it will mean that there are no nice solutions to the 18 players 6 round problem.
Let me know the number of rounds you would like to have with 18 players and I will make a schedule for you.
Ian.