It shouldn't be incompatible with Sleheyron Story Mode as the head developer, SAO, tries to make it compatible with RC-K1CP. Of course, there have been historic problems in the past which has made it incompatible with RC-K1CP, though in those instances those incompatibilities where due to the unforeseen bugs which later got patched.
For Sleheyron Story Mode, I would recommend asking the Sleheyron Story Mode devs themselves as they're the ones responsible for compatibility with RC-K1CP. The latest patch fixes an RC-K1CP incompatibility but if you were to, for whatever reason, discover a new incompatibility you would have to report that to SAO on the Sleheyron Story Mode page.
Whilst I do agree that BOSSR is an amazing mod, it's simply too big and too old of a mod for me to worry about compatibility wise, this is the same philosophy as the K1CP devs as well so it's possible it might not even be compatible with K1CP and if it was it's purely by coincidence and not by design.
That would depend on exactly what it is you want to see and how complex that bit of restored content is.
For the longest time people wanted the Echani in RC-K1CP and they have since been added, in terms of very possible restorations the Iriaz and Outcast Children are my top priority for this mod.
Long term, the Pazaak Tournament will be added as I have received permission from Fair Strides to port it from K1R into RC-K1CP. The RC-K1CP's version of the Pazaak Tournament shall be different from K1R's version and The_Chaser_One's version.
As The_Chaser_One here is a Sleheyron Story Mode dev, I would trust his advice on both BOSSR and Sleheyron Story Mode compatibility with RC-K1CP.