Flying Chemist
I was in a partnership 3 three other pilots in a C 180 on floats. I have a lot of float time but the other partners only had brand new float ratings. We got two quotes from the insurance broker. One for a partnership with 4 partners with my experience and one for the actual situation of 1 experienced and 3 inexperienced pilots. The later quote was about 50 % higher. I then paid a contribution equal to 1/4 of the all experienced pilot quote and the other partners split the rest of the premium.
My experienced is that for light aircraft (especially other than very common Cessna's and Pipers) insurance adjusters do not seem to be very knowlegable.
What is safe and sensable and what the insurance guys want are not necessarily the same. For a relatively unusual aircraft like the Chipmunk it is essential you get instruction from an experienced instructor with lots of time on type. There is more to a checkout than just getting good enough to not crash. An experienced instructor will provide invaluable operating tips, operating pitfalls, maintainance advice etc.
You mentioned your partners are experienced but did not say if they were experienced on type. If they have no chipmunk time than they will need the same training as you, although probably just not as much of it.