pHATs don't have EEPROMs, which is why they're called "pHAT" instead of "HAT" (to call them "HAT" they must have an EEPROM)... it's a very narrow line we're toeing, but it seems everyones comfortable with it. The whole EEPROM thing still doesn't really seem to have panned out into anything useful anyway.
You could easily use any pHAT in combination with any HAT, since one wont have an EEPROM and one will, providing the pins they use don't conflict.
I've also used multiple HATs together, but I've never looked at what mayhem happens to the EEPROM read when booting with two attached. I'll have to check up on that!
Ultimately it's electrically safe, but weirdness may occur!