You are not adding the fish until you get back. That is when you need to test daily. I would not waste my time/money on commercial bacteria. If you get ammonia after you add the fish then use the internal filter.
The ideal drip system will use well water (no chlorine) or have filtration to remove chlorine before the drip goes into the tank. Although I can see if the drip is slow enough the "bacteria" might be able to process it fast enough.
Water changes do not harm the beneficial bacteria. If nitrate has been accumulating as it should when the "bacteria" are healthy, there is no reason to skip or delay the water change. You might skip vacuuming the substrate or scraping the glass or cleaning the decor however.