I want to use a Mesa IO card

Since I have five Step motors and 1 or 2 DC servo motors (they use step & dir input also)
perhaps the Mesa 5i25 with ether two of the 7i78 or two of the 7i76 daughter boards
would do the job for me.

If for some reason I should need a few more IO lines can the computer par port still be
used in conjunction with a Mesa board setup?

Probably a 7I76 and 7I78 or 7I76/7I75 combo would do (and be cheaper than 2x 7I76)

There's no problem using a parallel port with Mesa hardware
(this is a feature of LinuxCNC, you can use pretty much any mix of hardware you like)

OR could I use two 7i75 boards to output my step and direction signals
I presume the FPGA is the actual source of the step and direction output
and encoder inputs are also dealt with in the FPGA.

2 7I75s will do the only disadvantage is that they are unbuffered so you only get 3.3V signals direct from the FPGA (like using a parallel port directly) You can also use parallel port type breakout boards with the 5I25.

The 7I73 (and 7I69 and 7I70 and 7I76 and 7I77) have remote encoder counters (not in the FPGA) but these are mainly intended for MPG use as they have no index and they default to 1X quadrature mode (so 100 PPR MPGs generate 100 pulses per turn or 1 per detent) They are also limited to about 50 KHz count rate.