What do you mean bottom 'corners'? I know of pectoralis major and minor. There's no way you can isolate the 'sides or corners' of your pecs, the muscles run horizontally across the body, and you're either working the upper or lower part of it. If you don't like the corners, thank genetics for that one. Anyway, if you want the bottom part to develop, you have to work the entire lower pec. Declines and angled dips may work it well. Though, what you were told, about the toning and mass building, there's no such thing as 'toning' a muscle. if you're looking for the toned shape, you're going to have to lose the bf% or get bigger muscles to show through. As for mass, compound movements are the way to go (ie. all pressing motions).