My 3 year (now almost 4) bunny weighed I believe just under 2 pounds when I got him. He wasn't fed correctly though but he is up to 2 pounds 6 ounces. It's winter here and he is eats like a pig with modifications to his diet.
I have heard that Alfafa hay is ok for young bunnies but not sure when to start cutting them off of that.
As far as the small dark poops my Bun went through a phase of the small poops when I first got him. I think because of being on a normal feeding schedule instead of barely being fed and getting better food. As far as Dark as long as they have the same consistency as his normal poops (same contents, firmness, ) as long as not almost black or black I don't know where there would be a problem. It may be his body adjusting to the new amount.
Did you try increasing by say half a spoonful at a time?