Cerastus kit question

you know how the the kits like the Questoris can make all of the different Questoris class knights....

Does the Cerastus kits work the same way?

and as long as I have you reading about the Cerastus knights, I'm a noob to 40K. the Cerastus kits boxes are all branded as Horus Heresy. Horus is a different game? is that correct?

but you can use those models with 40k? I have their stat cards and people use thim in competitive lists. so it would certainly seem they are "legal" to field.