11k and 11v are safe even for client that don’t support them. CCKM need client to support it and it’s almost deprecated for the successor 11r, my advice is to have a test WLAN with the same 802.1x WLAN you have but enabled 11r for mixed mode (meaning enable FT and check 802.1x and FT 802.1x check boxes) and test all your clients, this setup will not allow legacy clients that don’t support or understand 11r to access this WLAN but will allow 11r capable clients and clients that don’t support 11r but understand it.
Also enable 11k and 11v for that WLAN, 11k and 11v will help the clients with roaming decision but for the actual roaming 11r will help them with that.
Make sure you have all your FlexConnect APs in the same Flex group in “connected mode” with “central authentication” and test…