Can Enphase M250 and IQ7 Microinverters be connected?

Yes.

Enphase micro inverters M250 and IQ7 can be connected in series on the same line:

With junction box between cables Engage M250 and IQ7 (ideally with an Engage coupler, but the Engage cable is handled like any other standard outdoor electrical wire, allowing it to be cut, stripped, or extended if needed),
If they are then connected to an AC box that has a Q-Relay (essential for IQ7).

Important point: for the commissioning of the IQ7 you will need a gateway Envoy-S.