You mean you get internet midi tracks and use them as is? And you already have keyboards, guitar and drums? Then what are you using the tracks for - strings, percussion, bass...?
I would think that you would have to heavily rearrange any tracks that you "borrowed" or bought. Take out the melody and any other cheesy sounding bits. When I use tracks I take out all the string, horn, guitar parts, and all the piano parts save one. So I'm left with drums, bass, and one piano part that I mix quite low - then I sing and play guitar to that. But maybe what you want are the string and horn parts...
But here's another question, why do you need tracks anyway? A few years back, I did a summer production of Grease with keys (playing left hand bass), drums and guitar. The following summer we did Jesus Christ Superstar and just added a bass player. Both shows came off quite well IMHO.
I also like the above idea of recording the parts you can't play live and then playing to those. And although I feel it would be even nicer to add the required musicians, I realize the economy speaks loudly in these situations.
Carrying an LS608 down the stairs, a woman says to me "can you call me a cab, I'm not having any luck" ... got to stop dressing like a doorman.