For your costume, I'd start by going off of what your character would actually wear. Is she a tomboy, meaning it takes a lot to get her into a dress? If so, then adding a skirt to your outfit may not work out for you. Also, work around functionality. It doesn't make sense for a mechanic to walk around in her working outfit with stiletto heels. Since you said this was for a winter con, maybe a jacket or coat will work better than (or with) a corset or vest.
If nothing else helps, then make the pieces you already have designed and then play around to see what looks the best with them. Drape a bit of fabric to see if a skirt works, or hold up a vest or jacket you already have to see if something of that shape or color complements the rest of your outfit. Laying it out will also help in seeing what type of shoes will work the best with the rest of the pieces.
As for your friend, having a character layout may help her (or you) design the outfit. Also, more of her preferences can help as well. For instance, is wearing a shorter skirt or breeches with a bustle and a fitted vest too feminine or restrictive to her? If so, you could go with something like full length trousers with a fitted shirt, jacket, and feminine accessories.