Do not change the filter. There is no way it has established yet. After two more months, rinse the filter in used tank water. After that, rinse the filter as needed--you'll see reduced water flow and know it's time to rinse. Only change the filter when it starts to fall apart.
For your water changes, one gal will not make a difference. Do a 25% water change (4 gals).
Thing is, I have a RO device at my workplace, where I am able to get water from, and I have an empty plastic 1 gal jug. I guess I need to go out and bu a few more. I try to do 75% RO and 25% tap for mineral content.