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The night lord books have made me want to combine loyalist parts to the new sculpts. Would this not work, at the least i could probably put chaos heads and shoulders onto tactical marines and put the loyalist parts on more chaosy guys. (but would they look too small or dumb looking next to the new csm?)

It depends on the age of the kit. The new CSM are close in size to most of the currently available 6e-vintage SM kits, but they'll be a bit too big for the 4e-vintage range or the 30k range.

the answer is "more or less"

you'll be able to fit arms, shoulder pads and heads on plenty easy but the new torsos are incompatable with the old ones.

Shoulders work fine, they are only ever so slightly smaller. Those with a rim around them are at least as big though if not bigger.
Heads work better on new sculpts than old ones if anything.
Backpacks also work, the difference is negligible.
Weapons fit perfectly from old to new.
Torsos work but will require some work, these are more noticeably smaller. Once you add the rest though it's quite passable.
You also can't do half and half, as the new ones are big enough to cause misalignments. But again, works, requires greenstuffing or some other way of hiding the faults.

Arms are too thin and short, at least those from FW, haven't tried with old GW arms yet.
Legs are likewise much shorter and thinner and don't really work.

That on the picture is a FW torso and shoulder. With the terminator head from the new sculpts.
I had to pad the arms as it was too narrow and it's still a bit awkward, might have to add some belt or whatever to hide the armpit.