New Tzeentch player, need advice my first army

[Magister]

Hi Chaos players, I'm a 40k Tzeentch Daemon transplant giving AOS a try....fortunately already owning quite a few demon models. I've learned a lot just lurking and searching threads on this forum, but I wanted to ask three quick questions to help out my army list tinkering. I've only played a few games so far.

First, I own models which I understand are more hit or miss in AOS (screamers, flamers, etc). I like to build all-comers lists and was curious if Daemon players tended to reserve a certain percentage of points for summoning in matched play?

Second, the Multitudinous Host battalion intrigues me and I've read all the threads I can find on it. As I understand it, using the battalion bonus in matched play, as long as I'm replacing slain models in a unit I don't have to pay for them. Am I allowed to voluntarily forgo rolling to add horrors to a complete unit, or add less than what I rolled to replace casualties? If so, how many horrors should a unit have to take advantage of this without getting wiped out? I was thinking something like taking (in a 2000 point game) three 10-man pink horrors for my battleline, then possibly taking the rest of the battalion using 20-30 man blue/brimstone horrors. I have no idea if that would work well, it's just what popped into my head.

Finally, a quick modeling question. I don't own any burning chariots but am planning on getting some start collecting boxes since they have so much in them. Is the disk on the chariot the same size as a standard disk of Tzeentch? I wondered if I could use a kit to convert a Herald on Disk if it's the right size (plus 3 blue horrors, plus an exalted flamer on foot, plus a couple screamers if I get creative with the cutting where they attach to the chariot).

I appreciate any advice from seasoned players. I've only played a few 1000 point games, but honestly have enough to field 2500 points easily. I also used several Age of Sigmar heroes as Heralds in my 40k force (because you can take them in handfuls and I wanted visual variety) so I even have a bunch of different leaders to play around with  Anyway, enjoying the game so far.

[Soulsmith]

It seems generally advised to hold some in  reserve if possible. Destiny Dice make it a lot more feasible that they're going to come on, especially if you have a LoC.

Indeed in matched play if you don't go over the starting model count it doesn't cost extra points. I don't know if you can only recover some not all, but you can elect to not split or create more etc.

Flaming Chariots can definitely be used for disks, in fact I saw it about a week ago but I am not sure where.