Mesa 7i96 Ethernet board

It should be possible to use 2x7i96.

But, the 7i96 has an expansion port for any of the "dumb" 25-pin cards, so it would be cheaper and simpler to connect a 7i78 step/dir board to the expansion port on the existing 7i96.
Alternatively you could connect a 7i76 (not -E) to the 7i96. That isn't cheaper than a 7i96 but gives you even more IO if needed.

Are you sure that a step-generator is the right thing for the Mosfet driver?
If you need a fixed pulse sequence then it might well be, and bear in mind that the Hostmot2 firmware supports table-driven step types up to 8 separate pins in a sequence 16 steps long.
linuxcnc.org/docs/2.7/html/man/man9/hostmot2.9.html#stepgen
(The table mode docs there are not very clear to me, and as I think I wrote them that probably makes them even less clear to everyone else)
You would need advice from PCW / Mesa on which daughter board would be best for such a complex step sequence. It might be the 7i85S.

The 7i96 has terminals and hardware for 5 stepgens, but the FPGA can handle more with the right firmware.

There is an existing firmware definition which attaches a generic 3-axis paralell port BoB to the expansion port. That might actually do all that you need, for almost no cost. (Steppers on the BoB, Mosfets on the 7i96 terminals).
It can be found in the 7i96 support file from Mesa:
www.mesanet.com/software/parallel/7i96.zip
Take a look at the 7i96_5abob.pin file to see the pin allocations of the associated 7i96_5abob.bit firmware.
There is also a 7i96_7i78 pin / bit there for adding a 7i78 to the expansion port.

If you want encoders then yes, 7i85 sounds good.

Why stepgens for MosFet's ? Why not PWM as it normally should be done.
You can change the frequency of the mosfet gate but after about 100Hz it will have no difference in the output voltage up to the limit of raise/fall time of the mosfet or the gate capcitance.
In my opinion you do need PWM, or PDM but that is another subject.
I am aware that you can use very short step time and that would change the output depending on the frequency, but i do not think it will be enough. Having to short of a pulse can cause the mosfets not to open completely and overheat.
As i said somewhere, been thinking lately about doing a wire EDM, but it never crossed my mind to use stepgens for power control. That is a new one for me.