I used to be in a band that had two live guitars, bass and four vocalists, and also a duo of guitar, bass and 2 vocals. Everything else on backing tracks created mainly from downloaded midi files that I altered and treated with real instrument samples/VSTs.
The best advice I can give that was the key thing I took from the experience is aim for consistency and make it sound 'live'. Choose one drumkit and stick with it, same with piano, brass, strings, organs etc. You will thank yourself later for it. If you change samples and tones for every song it will play havoc with your mixes and your whole set will sound disjointed.
On the subject of mixing, try to base the volume around the drums if possible. i.e. try to keep the volume of the drums consistent across each track. Then mix the other instruments around them. If it helps (and it did me) lay down a guide bass that can be muted later. This will ensure that you leave room for your live bass to sit in the mix.
Lastly, some basic mastering. Mix all your tracks down to a mono or stereo* wave/aiff file (whatever uncompressed format suits you best) and load each one into a separate track of you DAW of choice. Then load a parametric eq and brickwall limter into each one (some prefer a multi-band compressor, but do what ever is best for you). This will allow you to get the best performance and maximum loudness out of the PA. Set the eq to roll of between 40hz-70hz. And then use the brickwall limiter to maximise loudness (without over compressing) and make each track an equal loudness.
In the bigger band this last step wasn't so much of an issue as we had a fair size PA and soundman to ride the mix. But in the duo with just a 250w fender passport PA system it made the world of difference. Previously, I could only turn up the backing tracks a certain amount before it started farting out, but after rolling the very low virtually in-audible frequencies off and maximising loudness at source, we could get far more out of our lowly PA. Not only that, but the volumes between each track was even and we didn't need to touch the PA after the first song.
*I'd also think carefully whether you really need stereo. Unless you have some real Jean Michel Jarre special effects going on, you really want to get all parts of the mix to all parts of the venue. Running a stereo track through a PA in stereo is also less efficient than running a mono track through both channels. The power is the same, but will appear louder as exactly the same thing is coming out of both channels. Also, if you're on a budget, mono PAs are cheaper than stereo ones!
