First of all, you don't need tongue on a Soulgrinder as it can just charge into combat with any tank or walker and mince them to pieces at S10, so take it off. You could replace tongue with phlegm to kill even more Space Marines.
The only big problem I see you facing is horde armies as you have nothing that does lots of attacks or shots. I would take blood letters or horrors for this job as a unit of 10 blood letters gets 30 A on the charge which is just mean, and with flamers and Soulgrinders and other nasties coming down the blood letters won't get that much attention. The advantage with horrors is that they would be killing stuff as soon as they come down and their Ap and S is fine to deal with mass armies and they still get 30 shots.
I would take off your special characters choices. Fateweaver just isn't worth it for that many points, as monstrous creatures should be charging into combat as shooting HQs just aren't that great and do much better in combat. It's only real good point is the re-roll saves but it's not worth the points or the HQ slot. Take a bloodthurster instead as he will fly around the board killing everything you want him to. Again with Ku'gath he isn't worth the points for his special rules, just get a geart unclean one as he'll do the job nicely and will save you points that you can use elsewhere.
Flamers, split them into groups of 4 or 3 as then you will get all the flame templates you need but you can put them in multiple places for the most killing. Also don't give them bolt of change they shouldn't use it and phlegm will take care of small tanks.
Finally the troops choices, don't take nurglings as they are worth nothing. If you want nurgle serviveability take plague bearers they will shrug off all but the anti-tank stuff that finds there way to them. As I said earlier take either horrors or blood letters they will make sure you can kill everything that you are put up against. Here is a modified list of what it could look like.
3 Flamers 105
4 Flamers 140
10 bloodletters 185
10 horrors 195