That's a pretty good list you've got there, there are a few things that could be better.
I suppose because you didn't put any in your list your soul grinder will only have his harvester cannon. Sure it's
str 7
ap 4 so not bad but I would either give him baleful torrent (a flamer
str 6
ap 4 with torrent which means you can put the template at 12 " from the loop of the gun) or Phlegm Bombartment (a
str 8
ap 3 ordnance, large blast). The advantage of these is that the Phlegm is has really high damage and the flamer doesn't suffer from the fact that the soulgrinder has only a
BS of 3 (which is the risk you take when using the large blast)
I would also change his mark from Tzeentch to Nurgle (the tzeentch let's you reroll 1's on invuln saves) but the Nurgle one is so much better (it gives you shrouded and slow and purposeful so you can shoot the phlegm and still walk normally). You could also give him the mark of khorne for the furious charge and make him a melee grinder (and you might want to consider the flamer in that case).
For your question about the Pink Horros, it's really only worth it if you can put them in groups of either 11 or 16. Since they get another warp charge if they are with atleast 11 and even 2 when they are with 16 so taking them in large groups can boost your shooting power alot. But take in count to not take them with JUST 11 or 16 since you'll lose the profit of taking that many as soon as 1 dies!!
For the rest you could consider replacing some extra troops for better upgrades on your princes (the
ml 3 on Tzeentch is really really strong) and you could try considering taking a Exalted on one of the princes (I would recommend taking a hellforged artifact with you like the Grimoire or Portalglyph)
This last thing is not nessecary but it's just one of your other options. I hope this helped