1) You can mix anything with Flourite. Up to 50/50, as the manufacturer suggests. But keep in mind the look of Flourite. I really don't think you'd want to mix a very light gravel with it, but hey, it depends on the look you want.
Several people on the site advocate sand. You could mix sand or another fine substrate with Flourite. But you will find over a period of time the finer substrate will settle to the bottom.
2) Re plants. Personally I would say the recipe for success is to forget about the aquascape for the first few months. Just jam as many fast growers in there as you can, get them growing well, and then start figuring out what look you want. The fast growers then can be slowly eliminated, if need be.
3) Take your time and plan things carefully. Spend lots of time here before you make any final decisions. A good starting point (K the c posted a good link as well) is www.rexgrigg.com
. Remember planted tanks are all about balance. You need to know what type of tank you want to have. I realized I wanted a moderately high tech set up. So I have > 3wpg, pressurized CO2, Flourite, heavy doses of ferts. It is nonsense to say you can't have success with a lower tech setup. But you must balance things out. You are looking for trouble if you put a ton of light over the tank and don't plan on adding CO2.
4) Some of us do 50-70% water changes weekly. There is also the PPS system where folks do less frequent water changes. Again, do some reading, do some more, figure out what method you want to use, then setup the tank. Personally I recommend the estimative index, I am somewhat of a newb to ferts, and have had success thus far. There is no one right way to do things.