Just to let you know, not all silkies are very fragile like other people say. The only type of silkie I see with more head trauma are the ones with Vaulted skulls.
They are winter, & heat hardy. Mine have survived the coldest of winters, & the hottest of summers.
There's rarely any aggression problems between different breed chicks. Them being the same age, is the best to add the new chicks.
It wouldn't hurt to get them now, if you really want too.
I don't have answer for vaccines, since my birds aren't vaccinated for anything.
I keep different breeds of chicks with silkies with no issues. Sounds like frizzled Easter Egger chicks are what you're describing.