The way to do that is to simply make Pursuit the default action, by removing the felguard conditional - so that if none of the other conditions are true, it just falls through to casting Pursuit, and /petattack.
So, it would be something like this:
Code:
/cast [pet:succubus]Lash of Pain; [pet:voidwalker,combat]Sacrifice;[pet:felhunter]Spell Lock;[pet:Imp, target=player]Flee;
/petattack
/cast Pursuit
This way, what will happen - is if any of the above conditions are met - (i.e. succubus, voidwalker, etc), it will cast the appropriate spell. However, if none of those are true, it will "fall through" to the default actions: /petattack /cast Pursuit.
I'm currently using this technique for my mount button - so depending on where I am, and what modifier (ctrl, alt, shift) key I'm pressing, it will call either the seahorse, ground mount, or flying mount. However, if I'm already mounted and click that button, it will dismount me instead. Like the above code, after all the nomounted, flying allowed, Vashj'ir checks, at the very end, I just added /dismount.