if you use pins 2 to 9 to provide the drive for 4 stepper drivers, you only have 4 more outputs
pin 14 to provide the VFD analogue control and
relays1 & 2 controlled by pins 14 & 16 - see VFD mode jumper settings -
for example
US mode
PWM on = RLY 1 on
PWM & REV on = RLY 2 on
and a 3rd relay that can be controlled by pin 1 or 17 depending on jumper setting
so the question is how many of the on board relays do you want to use and how ???
if for instance relay 3 is controlled by pin 17
you could use pin 1 for your SSR
depending on how you have set the COMMON terminal for your stepper drivers
you will need to connect the SSR in one of 2 ways
John
PS
if you do not need to reverse the spindle
by using the international jumper setting
pin 16 that controls relay 2
could be used to control your coolant pump