SFP kits come with license key paper inside the box, so even if you buy generic SFP you will need license key to activate SFP port on the radio which we do not sell as individual parts.
For the fiber SFPs, other SFP modules of the same type can generally be used. We don't offer any guarantees about this.As Sagar points out, you will need the upgrade entitlement, and most likely the extended gland from the kit.
For the copper SFPs, only the exact module can be used.