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So a some of you may or may not know, essentially all my Marine bodies were the result of several purchases (I'm talking 4 boxes) of Burning Of Prospero because Mk3 über alles. While I managed to sell off 3 of the kits for the Sisters (and keeping one for myself for casualties on bases or just getting killed off in general), I've been keeping the Custodes for a second army, and now it's time to sorta start planning that (the Marines aren't finished but the plans are finalized at least). Now one thing I absolutely hate is the over the top ornate nature of the helms and sorta the weapons, but the basis of the model itself is fine. I was recently inspired on Instagram seeing someone use Stormcast Eternal heads on their models and I thought to myself "This guy is a genius". For reference I saw this:
https://www.instagram.com/p/CGm1THNn_bs/?igshid=1wzy2qwxdbspi
So my general questions are as follows:
1. It seems like the head compatibility is pretty easy without a lot of work, whether it be the Terminator Custodes or regular. Are Stormcast heads pretty cheap to purchase?
2. Are any of the GW kits really compatible with Custodes, or would the work simply not be worth it? For example, Custodes Sags would probably look nice with the Stormcast Hurricane Crossbows via removing the obvious crossbow reference on the weapons.
3. I don't see a lot of 3rd party bitz, if at all, referencing Custodes. Is that really not a huge market compared to the other armies for demand?

As someone who does a looooooot of converting across Custodian, Marine and Stormcast lines, there's a lot of intercompatibility. Custodes and Primaris bits are effectively interchangeable. This includes Gravis and Allarus variants. You may need to fiddle a little bit here and there to get a seam to match up, but it's very low key.

Stormcast...are a bit trickier. The way the shoulders are sculpted varies from kit to kit and can require significant clipping or cutting for the ones that have multiple "layers". If you look at the Sequitor kit for instance (and this is fairly representative of the Stormcast line as a whole) there's two sets of shoulder armor layers, and the lower one is sculpted onto the arm/torso. You'll need to clip that one to get Marine/Custodian shoulder bits to seat correctly, plus or minus some greenstuff (or your preferred variant of gap filler).

The other difference for Stormcast is that there are female models in the range, and the torso proportions are very different. It's not insurmountable, but if you end up using them you're going to need to do some bulking up of the under arm and shoulders. Fortunately, if you're using Marine pauldrons, nearly all of it will be invisible to anyone who isnt looking straight up into the armpit. Finally, the heads (and head sockets in Stormcast torsos) are ever so subtly different sized. Stormcast have thinner, but longer necks (the ball in the ball and socket connection that GW does for heads into torsos) You'll need to do some clipping around the neck armor areas on some, otherwise you're going to have *very* restricted head posing options.

All told, they're not insurmountable obstacles in my opinion, but they do require patience and planning. They're not quite as plug-n-play as going from Primaris to Custodes (or vice versa). Completely worth it in my opinion though, there's gorgeous stuff you can pull off.

I have over 100 Custodes Infantry, and about 40% of them are based on Stormcast. Custodes arms fit on stormcast bodies (at least the Eternals) pretty well. You can put stormcast shoulderpads on custodes arms, but not the other way around (without cutting). Many of my conversions are with the Easy to Build Stormcast eternals, and all I did was swap the head for a Custodes helmet and use the sword and shield option (it hides some of their AoS origins behind a giant shield). I've had players not even notice they were conversions before.

This is great info to have. I know the kits are near the same size so I'm not too surprised about some cross compatibility here and there.

Doesn't seem like much info regarding 3rd party weapons though, huh?