This is in my experience, so ymmv...
Make sure you actually have enough coop/run space for 6 birds, especially if some of the birds are a little "special" a lot of the coops you see are too small for the numbers they recommend.
The younger they are the easier it is to integrate more birds, so if you want to get more, the sooner the better.
I assume you are looking for more hens, not a rooster, generally you can integrate a bantam or special rooster with standard hens with no problem, but more hens can be iffy.
It does depend on your individual birds and their temperaments... of the breeds you have I would worry most about the RIR, ime, they are usually the dominant bird and they are not benevolent dictators. Depending on the birds you have you may have to select additions based on how they will probably get along with one or more of your existing birds... so look at your girls and try to figure out who the trouble maker is, and who her best friend is, and look for a breed that has a temperament similar to those your dominant bird gets along with best.
Frizzle is actually a feather type, so if you get something frizzled similar to what you already have, that is probably your best bet on a drama free addition.
I'd be worried about head-feather plucking with polish or any type of bird with a lot of head feathering when in a coop or enclosed run situation with other birds, it is pretty much inevitable. It does not seem to be as much of a problem when they pretty much free range.
Silkies have a lot of size variation so if you go with a larger one they may be OK, but the girls seem to be fairly submissive and that may be a problem with the standard hens you already have.
Cochins are generally really big birds and I have not had much problem with them and other breeds, they do have a lot of foot feathering which is a nuisance if you have a lot of wet weather or mud (Silkies are the same but not as bad).
Bantams and Standards generally wind up with the bantams being picked on ... though I have had some that got along well with standards and hung out with them, they all tended to be bantams that were pretty dominant girls.
You might also consider a standard breed with more of a fancy feather color, ie Speckled Sussex etc.