18 players, Gs should be a 4th of the civilians, but 18 is bit weird, should go for 4 or 5? Its between 16 (would be 4 Gs) and 20 (5 Gs) exactly, hehe sound pedantic, either will do.
And good question on when to start, but is the deadline time settled? 20.30? Way soon, but I guess its a must (with special roles too, settle a deadline for jailer, earlier in the day to adapt to night time deadline change)..