802.11r MDID openwrt VS other vendors

can someone tell if openwrt fast roaming and other vendors fast roaming is compatible?
specificaly, i've got aruba APs where the mobility domain ID can be just 1 number , vs openwrt which needs 4 hex digits.
is it enough to convert a 2 digit DEC number into 4 digit HEX number , or is there more complications than just that?
also - can the MDID be the same along 2 different ssid?
i have 5ghz ssid on 3 aruba aps sharing the same MDID, and now would like to add a 2.4ghz network with different ssid to the same mobility domain.

reason is - aruba is OK inside the house, but not outside. for outside i have tp-link 1043n which covers the whole property but the phones first connect to it, and then keep stayed connected to it instead of roaming to 5ghz on aruba.