Re: Rainbow playset question...
Well, we don't have a Rainbow(couldn't get a tube slide with the system in our price range, my 9yo's only request!) but we decided based on how tall kids "normally" are and not get anything under 5ft. Which I believe all their systems start at 5ft anyways for platform height.
I guess you need to decide what the underneath will be for. We needed ours to be able to fit a plastic playhouse underneath(I know some systems come with tire swings underneath, so you gotta think clearance height), along with a taller plastic climber for my littles who can't get up into ours yet(like 18 months and under since I do daycare).
So we went with 2 6ft towers with a 10ft tower in the middle(3 total tower system).
There is no single RIGHT answer, only what works for you.