No, your rocks/gravel should be fine, no need to add dirt :)
Smaller rocks/gravel just make it easier for the roots to dig down and become rooted. Enough fish poop and leftover food should get between rocks for the plants to breakdown into plant food...most hobby aquarists that just have a few plants, don't even need to get plant fertilizer (which is fish safe). Yes, continue to clean and vaccuum tank on regular basis.
No, you do not need to get a special water conditioner...what you use for your fish is safe and perfect for plants.
I can only think that you have preconceived idea of plants making a messy tank by watching all that anarchis melt away in dad's tank back in the 80's. I watched this happen, so I might be a bit anti-anarchis. If it's unhappy, it will shed and melt all over....
Otherwise, it might interest you to know that plants don't make a dirty tank...they CLEAN it!
Plants breakdown and eat fish waste such as ammonia, which we knows will kill fish in fairly small quantities.
If you want to keep away from the messier plants, don't get any fine leaves plants...if it looks like a for tree, its not for you...they are more difficult and they usually like CO2 injection and high lighting.
I would stick with java fern, anubias, hornwort...I would also add most crytocoryne (crypts) species to the list as the are inexpensive, slow growing plants that take much abuse :)...try and get rooted plants that say "low light" ...if you are using a light that came with your tank, it is low light...even if you get a "plant" bulb, it is low light.