And something in the colors to tie them in as well as styles that complement one another! I think that would look great!

I can't quite imagine it ever working, although I suppose if you're careful with the color of furniture involved and particularly careful that the leather sofa isn't too modern looking or too masculine looking it could work. It may also work if you keep the chairs far enough away from the sofa that it doesn't call attention to their difference and instead kind of makes it more of a "here's-this-part-of-the-room/here's-that-part-of-the-room" type of thing.

I think you may need some "minor" furniture or accessories (in addition to any pillows) to either pull it together or else define each section of the room more as well.

(Some wing-back chairs, as you know, are a little less feminine-looking than others, so that's something to watch for too.)

It can totally work! Can you please edit your question and add pictures of the type of couch your considering and colors? They make leather wingbacks (see source) so why couldn't leather and wingbacks work together? Your idea to add pillows is a great one. You could even add a leather pillow to the fabric chairs (or fabric pillows to the leather couch - but I wouldn't do both)

It will look beautiful! Just try to get the sofa to be similar in style. If you can have pillows made from the same materials used on your chairs that will really tie it all together. Don't even hesitate, you've got the right idea, go for it!

Sure this would work. "Matchy - matchy" is booooring. Ecclectic looks show creativity and are much more interesting and comfortable to live in. Just a few guidlines though. Leather has a somewhat masculine feel, so with your chairs, avoid fabrics that are too light, delicate or feminine. ( ie satin, delicate florals, pastels ) Don't do them in leather either, choose fabrics such as tapestries, velvets, stripes , heavier prints in bolder, deeper shades with detailing such as nailheads, button tufting, contrasting piping or wood trims. The scale of the chairs should be heavier also. Use the same chair fabric for sofa toss cushions or a throw blanket, and the same leather for 2 chair toss cushions. Choose a leather colour other than black or brown, such as a rich burgundy or a caramel beige that can also be found in your chair upholstery. Have 2 footstools made in leather with the chair upholstery on the tops. Pull out any or all of the other shades in your chair upholstery and use in paint colours, draperies, area carpets, art, etc. Don't be afraid to mix patterns, as long as your colours are consistant. Be bold, it can work!