Hey J,
I would generally use 1/2 for a 5 gal batch. For 3.5, I would use just under a half.
Ill effects from using too much will cause a fluffier trub thus making it less dense and harder to drop out of suspension. Less is more in this case.
Irish moss is normally used at 1 tsp per 5 gallons...
Never too soon to move to all grain. Only thing holding you back is your equipment. With enough will and help from these guys...you'll probably wish that you had made the jump earlier. I was bored with extract after my 2nd batch and was determined to make the jump right away. My 1st AG batch by...