redufresne, it is normal for a chick to have a bulging crop, if it goes down by morning when the food has moved from the crop to the gizzard and into the intestines. Some chicks are little piggies, and you can be astonished at how big the crop gets!
You do not need to force olive oil - that is good when there is an impacted crop. If the chick has an impacted crop, which sometimes can happen from eating shavings etc, then the bulge would still be hard in the morning. I say morning, if you are using the heating pad and they aren't getting light all night long, they won't be eating all night and there is time for the crop to empty overnight. I don't think from what you are saying that there is an impacted crop, just a chick with a big appetite!
aart is right, you can over do treats and it is good to try to keep treats at 10%.
mash is not the same thing as crumbles. Mash is a ground mix of grain, corn, and other stuff - you can see some of the different ingredients. You can feed mash wet or dry - if fed dry you end up with powder left over and it sometimes gets wasted. With mash, you want to have chick grit (tiny gravel or some sand) available.
Crumbles are a manufactured, highly processed food which is all one color, dried. You don't need grit for crumbles because there is nothing that needs to be ground up in the gizzard - it is the consistency of flour as it is finely processed.