registered has nothing to do with ECC support.
most mobo's i believe support one only type, most being non registered variety. so no mix and match
mobos that support ecc, can use non ecc memory, and you could even mix and match ecc and non ecc, but your computer will run your ram as non ecc.
mobos that don't support ecc, can use non ecc, ecc, but it won't run as ecc.