this is the first time i see megami device body parts mixed with shinkis... i never though they would be compatible to begin with!
so i ask: is there any modding involved? if not then are any parts "forced in" (like with stress marks or something)?
Most parts fit together without much of an issue (the level of compatibility is around 80%, with only a few things being completely incompatible, and some oddities like Megami Device arms not fitting on Shinkis but Shinki arms fitting on Megami Device bodies), and hold together about as well as Megami Device stuff normally does — which is a bit unpredictable tbh; the custom I showed off exists more or less entirely because I got sick of trying to fix Megami Device torso joints, which break really easily (though they give a great range of articulation before they inevitably break) — but the arms, head, and upper body need to be forced a little, though not to the point of causing actual damage. (Which shouldn't be surprising, as removing Shinki heads from their pegs or separating upper and lower bodies requires a fair bit of force to begin with.) I think MMS3 arms fit better on Megami Device bodies than MMS1 ones do, but I might be misremembering (I don't have any spares on hand to test with.) No parts need to be modded for what I've done here, though a bit of modding could probably be done to use chest pieces from one line on the other. Shinki skin tones are difficult to match with Megami Device ones, but this could probably be fixed with paint and sometimes they _do_ work pretty well together.
Also note that most of what I've done has been with MMS1 parts, as I have piles of spare MMS1 parts but very few MMS3 ones, so YMMV. (Also I couldn't figure out where to put this, but Megami Device hands will fit on most Shinki arms... just not as well as Shinki hands do. They'll hold on just fine most of the time, but might fall off if you're not careful while moving or posing the figure.)
I'd honestly actually recommend using Shinki lower torsos with Megami Device upper torsos, as Shinkis have better leg articulation and a less fragile joint connecting the two, albeit slightly less articulation in other places (also, don't remove the little insert from the shinki lower torso's ball joint, as it doesn't fit properly if you do that), and Megami Device kits have better shoulders (and cool double jointed legs that are actually slightly wider than Shinki legs but not to an extent where they feel out of place as a result).