if you have all those models you can build most effective army list

, if you want build a multi threat list you need diffent approach
for example 3 plague drones without herald aren't a treath, same for 5 fiends you need heralds to make them effective, slaanesh herald probably is one of best hearald you can have, and you need hounds they are probably the best unit demons have, fast hit hard decent resistance.
for example i played a thing like this
herald
SL 2°level exalted locus greater reward steed of slaanesh
herald
SL 2°level exalted locus greater reward slothful claw steed of slaanesh
herald
SL 2° level exalted reward steed of slaanesh
herald of K juggernaut lesser reward exalted locus
8 fiends
2x11 horrors
15 hounds
15 seekers of slaanesh
7 screamers
lot of fast wounds, 10 CW usually i get invisinility/shrouding you can remove seekers for another unit of screamers and make a large deathstar with 8 fiends and 3 heralds.