Yes, a cochin would be a great buddy for a silkie.
My broody partridge pullet has now hatched her 3rd chick. Found the second this morning and when I went out after work to get it to put in my brooder there was another!!!! I have them in the brooder with 9, 11 day old Mille Fleurs. The partridges are 1/2 the size of the milles who have been eating like piggies and are growing like weeds. The little partridges just dive right in and try to snuggle up under the bigger chicks. I don't think the others even notice.
My neighbor gave me 3 Black Copper Marans to brood for him and it will be neat to watch the differences in growth rates. Having strictly bantam cochins myself, the "regular" sized chicks and eggs look huge.