Silkies?

Three would be even better! Yes, Silkies do need buddies of their own kind in my opinion. I would worry too, that your older pullets will pick on them. I keep all of my Silkies seperated from the other breed I have, Marans, so I don't have to worry about fights or them getting picked on. I say fights, because the Silkie roos don't see themselves as little, and will challenge a roo three times their size!!!  By the way, if your girls are just 20 weeks old, they are still considered "pullets". A female is not considered a "hen" until she is a year old.

Most definatley 2! Even though you are nearly at your limit, it will be worth it. Plus you could get different coloured ones :) 

Good Luck

:)

i tryed putting silkies in with my BIG Rhode Islands. it didn't turn out good. the reds are so rough, and silkies are so fragile. i keep standerds, and bantams apart anyways. just because

I have the same problem, we keep our Silkie in the front garden because she was nearly killed her first time in our coop- (we have 3 amaracunas and 1 cochin) At night she sleeps in the coop in a small dog crate.  All our birds are around 5 or 6 months old.  I am going to get another silkie for her companionship.  I am most worried about her sleeping at night.  This is my first winter with chickens and I am afraid she will be too cold on her own.  I have set up a divider in the coop to separate her from the other chickens.  I dont think there is much you can do but keep her seperated on her own or with another mate silkie. good luck!!
 

ANY ime you add new chickens, regardless of breed, there is very likely to be disruption in hte pecking order, and some scuffling/fighting.  There are things you can do to minimize it, though.  

Put the new bird(s) in inside a wire cage so that they can all become accustomed to the appearance and sound of each other without physically interacting.  

After several days or even a week or so, provide some limited supervised time to interact with each other.  Preferably in the late afternoon/early evening, a half hour or so before they head to their roosts for the night.  Even if they get along fine, pen the new birds up again for the night; give them several successful "play dates" before turning them loose together.

My silkies tend to be more aggressive than my largefowl, my polish, my brahmas, my moderns.  Generally the smaller the silkie, the more likely she is to be top hen.