Most people don't like to miss match fabrics. If you can afford a leather furniture set than it probably is best to not throw in you mothers brown and yellow arm chair from the 1960's. If you have the space than a L-3 piece couch set is an awesome addition. If you plan on entertaining guests than seating for at least 3 or 4 is essential. You sure as heck don't want your guests sitting on the floor!
The idea of the coffee table has somewhat changed. Iv even seen someone put 2 clean car tires down and a big sheet of glass on top and made the coffee table look wonderful in their home. I think its neat when people get creative like this. Glass coffee tables are not such a good idea if you have children, for a number of different reasons, one they can break and two, they usually have sharp corners. The perfect coffee table for small children is a round wooden table with padding around the sides. LOL
Finally, when adding to your living room furniture, be sure to remember to be creative. Don't be afraid to add a bright red chair in the middle of 2 leather couches to add flair to your living room. Have fun, and most importantly don't ever buy furniture on credit, if you can't afford the furniture the day you buy it, you probably can't afford it period!