My experiences with wyandotte have not been good. Such a shame all mine were mean to every other breed.
Other people keep them quite happily in mixed flocks.
I would be careful of bringing in a lone silkies with RIR and wyandotte. Thin of it as the silkies re the skinny freckle faced kid and the others are the bully kids. Not fun to be the one different kid in the class.
Are you prepared to house them separate if things get crazy?
Personally, since silkies are broody a lot, look different then the big birds, and are quite small I would keep a flock of bits and a separate lock of silkies. It is hard for them to see well and that gives feather pullers an unfair advantage.
Same with Polish for me. Even my docile hens took note of the head feathers.
Can you get 3 silkies and the 3 Sussex without getting wyandotte?
My Sussex have all been mellow and sweet birds.