Think from an end user standpoint. If you are to mix it up like that, make it desirable
ABC1 - Acceptable
CAR2 - Acceptable
SEX3 - Acceptable
HOT4 - Acceptable
RED5 - Acceptable
ZVW1 - Unacceptable
DFN2 - Unacceptable
SXB3 - Unacceptable
GHH4 - Unacceptable
LCZ5 - Unacceptable
If you had the likes of a "HOT4 DOT NET", you'd be on shaky ground...COM only for the majority of such combos. IMO