1) Using a timer with your venturi pump will be fine. Have it turn off when the lights do, you don't need it on after the lights are out. I would have it go off when the pump is off.
2) You might want to use 2x 2L bottles, I'm not sure. Somewhat related to what Glenn said, venturis have a "suction" to the venturi line. If this suction is great enough, it could potentially (not likely) suck some of the DIY mix into the tank. However, let us know what venturi design you have (pictures) and we'll be better able to help you.
3) I use champagne yeast. This is the first hit I found on Google. I got mine on eBay for cheaper. Look for 10-packs.
http://www.fermentationtrap.com/cb4904.html