I've been playing a LOT of Nurgle this edition thus far. I'll share my 2 cents....
GUOs
If you have two models, go with them.
GUOs are excellent for their points. They do die to massive lascannon fire, but I've had them completely wreck face multiple times. If I owned two, I would likely be running two.
My only worry with running 2
GUOs is that only one can heal per turn in matched play. In one game, I healed a single
GUO for 10 wounds during the game. When you are splitting that out, it's not quite as awesome.
I've ran a renegade knight with my
GUO in the past to good effect. This weekend I'm going to try Scabeiathrax the Blighted.
Plague Drones
Plague drones are fantastic units and one of the most under-appreciated units in the codex. Their value is not immediately visible on the statline, but becomes much greater in the game.
They benefit greatly from a prince flying in the middle of them. He gives them extra punch, and he also increases their chances of hitting.
Virulent Blessing is fantastic on Plauge Drones. It increases their damage output to 2/4. To make it even better, the reroll is applied before the +1. This means if they are attacking a rhino, for example, and you score 18 hits (not unreasonable rerolling 1s) you have a distribution like this. Your first set of rolls will produce 3 six's, 3 fives and then the rest will be failed rolls. You pick up those 12 dice and reroll them. Out of these, you get 2 more six's, 2 fives, and 2 fours. This means you got a total of 5 six's, 5 fives, and 2 fours. 5 of the wounds will cause four damage, and 7 of them will cause 2. When hitting something like a rhino, those five wounds are nasty.
Plague drones also have a large base, making them great for blocking in movement. I've used them to trap models from getting to objectives multiple times. Since they can fly, they ignore enemy models for movement, meaning you can charge a unit, and proceed fly over them and block their movement as part of your assault.
Plague drones have assault guns, so can shoot them after advancing.
Plague drones can fly, meaning they can leave combat and shoot their guns when needed -- which I have done.
Run your plague drones in units of 7. It makes Nurgle happy.
I run one unit. I've found that to meet my needs for the drones well. Let me know how 2 works for you.
Nurglings
3 stands of nurglings is just begging to have first blood taken from you. Personally I would not run this -- but your mileage may vary.
I run a squad of 7 of them in my army. Again -- Nurgle's magic number.
I used to use them to put heavy pressure on enemy units, but now I've been using them to place just outside my army's deployment zone to be in range of the herald and to stop deep striking alpha strikes.
A squad of 7 of them is much harder for someone to kill in a single turn.
Plaguebearers
Keep them at 30. Add an instrument. They are great units for holding objectives. In one game I had them strung along to grab 3 objectives, since units are not limited in this edition to only owning one objective.
Again, it's one of those units that can do more than they do on paper. I've had two cases where a big assault creature (hive tyrant and
DP) assaulted my plaguebearers only to get killed on my turn in the assault. Plaguebearers are weak against units that can deliver a ton of attacks, such as bezerkers, but against units that hit a few times really hard (like a
DP) they are hard as nails.
Yes, I've had an entire squad of 30 plaguebearers killed in a single round of shooting -- but this happened with my opponent spending his entire army shooting at them with a Tau shooting army. That has only happened once.
My Own Experience
This is just my own experience. You may have had vastly different experiences. For example, I've found soulgrinders to be really lackluster -- though people like
S.e.r.a.p.h.i.m have had great luck with them. Basically it means that everyone finds what works for them
~Ice~ 
