when my silkies go broody im planning on buying some eggs ( pekin bantams ) if i put them under her will she hatch them and care for them or not ?

Silkies are renowned as broodies but I would wait until spring before getting any fertile eggs.

Have you got plans in place for any male chicks that hatch? (Why I keep a sexually dimorphic breed so chicks gender can be determined on hatching.)

Yes, but not more than 5 or 6 eggs, the silkies being rather small to cover them when they hatch, they tend to be good mothers as well. I currently have a little duckling (chuck) with mine, and they do not even know the difference.
she hatched and cared for 5 ordinary chickens that were twice her size as well. pets question and answers,www.5d2d.com

good luck