What would be the slowest shutter speed possible while using a monopod and would it make more sense having IS turned on while using a monopod for slow shutter speed?
There is no such thing. Unlike a tripod, a monopod only improves stability. How much will depend on the particular monopod, its height, the wind, the focal-length you use, how you breath, etc.
What you can hope is that it will improve compared to how you hand-hold. Remember than these is no absolute their either. A common rule of thumb is 1 over the focal-length but recently many photographers claim 1 over twice the focal-length or more. That is because increased resolution requires more stability.
With good breathing, a solid stance, a short focal-length and stabilization on, it is possible to hold 1/4s or a little less but I am not sure at all how this can help you.
In regards to stabilization, yes use the stabilizer as there is still movement to stabilize with a monopod.