Can I mix a leather sofa with a fabric love seat? The pieces are from the same compAny and the same style with the same hardware, legs, etc.

I would not hesitate to use them in the same room. In home décor magazines this look would be tied together with accessories chosen to coordinate with the colors in both pieces. Iron / Mediteranian candle holders and end tables would go with the leather if you like that look. Use the colors in the love seat on mats for pictures on the walls, and bowls or vases on coffee tables and end tables. Use these love seat colors for throw pillows on the leather sofa. You may also use shawls or throws in appropriate colors. Room draperies and/or blinds should be in colors that coordinate or are neutral. If you collect anything, display them on shelves or in cabinets, but do not use too many and thus make the room look too "busy".