Technique forum would have been better.
Your guitars may play in 'dropped C' tuning, but why would you need
to change your tuning at all on a 5 string to accomodate them?
Your string gauges are perfectly fine
the way you are tuned normally BEADG and your low C is
available on the first fret of the B. Why wouldn't you just
play your bass from that pattern. Who cares how the other
instruments, guitars, cellos, sax or trombone are tuned? As long as
you are playing in the same key.
The 5 string gives you this advantage already without changing tuning.
Just becuase you guitarist needs to tune down, you don't have to,
you are already down to 32 HZ, no need to go
any lower or change your scale at all.