will my temp control plan work?

i am a poor college student and barely managed to scrape together a keggerator and pay the extra electric bills. i keep hearing that i need to control my fermentation temps and i want to try. I was thinking since its the very beggining of fermentation that gets the hottest and has the most activity could i just control the temps during primary fermentation and then leave it to secondary ferment or age in the air conditioned room like i used to? i know ideally i would control it the whole time but would i see great improvement if i just controlled primary temps?

Also i was thinking of just putting my primary in a cooler filled w/ water and just check the temp and add ice/warm water. would this work or would it be too great of a temp swing?

will work fine. i used it over the summer here with 20oz bottles of frozen water. during the day the house can reach temps in the 80's 1 single bottle of water would keep the temp to 70* all day.

Lots of people use the water bath with ice bottled trick and it works well.

One added way to keep it cool/reduce space is put in in a tub with water coming only 1/3 the way up the carboy and drape an old t-shirt over the fermenter so that the bottom hangs in the water. Wet the shirt with the cool water and check it every day or so to keep it moist. This will wick additional moisture up the carboy and give it some evaporative cooling.

My self and others have reported an additional temp drop of 6-8 degrees F from the carboy to the ambient temp.