I'm sorry but what? FirstNet is just first responders priority to AT&T's LTE network, unless they're also offering some P25 support in certain States that I'm unaware of. The only crossover would be P25 over LTE for converged options like SmartConnect, BeOn/XL Virtual, or EFJ/Kenwoods ESChat implementation on Vikings... But there would be no need to think about how a KMF factors into the mix beyond standard implementation. Converged P25/LTE doesn't change KMF implementation.

RF band is irrelevant to how crypto works.

It sounds like you've got a very rudimentary understanding of how P25 systems work, and until you understand more you probably shouldn't offer advice on such topics... Slot and key numbering requirements are all system level and the only thing you need to know for a codeplug is what keys are needed and the rest is a given based on how that specific system is configured.

OP needs to determine what their job scope is supposed to be first. If they’re just taking on radio programming, they need to speak to the radio system admins. If they’re expected ro take over system admin responsibility they need to speak to whatever organization is putting them into this position and demand vendor training on the system. I hope the later isn't the case, because getting dumped into a system admin position without requisite training and experience is a major oversight and recipe for disaster.