Yes! what you've said helps a lot!
I've talked to some friends and acquaintances about this as well and they let me know that I should pick up a DC Variable Power Supply if I'm going to be doing any electronics anyways. I ordered one and it should be here soon. Then I, as you said, I can test the pump under different loads.
Another suggestion was made to me to put the pump on a PWM connection and allow the Arduino to control the maximum load to the pump. Is this feasible?
Some other suggestions: As you said, get a bigger motor, don't use a 9v battery anyways, don't use resistors, etc. I think I would like to switch over to using the appropriate amount of AA batteries (or possibly C batteries) to get enough voltage and mAh to run the board and the components for a while.
I've seen that the Arduino runs at 25mA while running code. Does that mean it will always be running at 25mA while it's waiting for a reading from a humidity sensor? Am I able to put the board on standby for a set amount of time? possibly hours? Then have itself take a reading and turn the pump on from there if it needs? If this is the case, then I would preferably want to run 4-6AA batteries in serial to get the voltage to 6-9v (hopefully I will only need ~2000mAh)