Typically, in the rock and metal world, a solo will be played using only one mode. Sometimes a combination of modes can be used to good effect as well.
For example, since you mentioned Slash, much of hs soloing is pentatonic based--but he does use combinations of aeolian, dorian, and mixolydian very often. However, I doubt Slash realizes he is doing this.
It sounds like there may be some essential theory you are missing in order for you to be able to use modes effectively in your soloing.
For example, which mode would you use to play over this chord progression: G, C, and D?
How about this one: A minor and D major?
Let me know if i can help!
Nick
http://www.nicklayton.com