The form of the feed isn't important as far as nutrients. To make feed they gather all the ingredients and grind them up. This is called "mash". To make "pellets" they add water to mash to make a paste, extrude it through a round die, and flash dry it. To make "crumble" they partially crush pellets. So just saying pellets or crumbles doesn't tell us what age group the feed is good for. The important variables are protein and calcium.
The reason they make these different forms is mainly because different automatic feeding machines work best with different forms. Baby chicks can't handle pellets so chick feed has to be mash or crumbles, but other than that the different feeds can come in any form.
I get Dumor from
Tractor Supply so I think I know what you are talking about. The 20% crumbles is their combined Starter/Grower. You can feed that from when you get them until they start to lay if you want. You could feed them that 20% forever and never switch, just offer oyster shell on the side for those that need it for egg shells when they start to lay.
The Finisher is their 15% Finisher/Developer, also in crumble form. According to the chart on the bag you can feed that from about 10 weeks until they start to lay. While the 20% Starter/Grower won't hurt them, they don't need the higher protein during this time phase and the 15% is less expensive, $2.00 per bag less for the 50 pound bag. The 15% is what I use. Don't switch to the 15% just because they reach a certain age. Feed it until the bag runs out.
The pellets is the Dumor Layer. I don't use it, I do the oyster shell on the side since I practically always have juveniles in my flock, but I think it is 16% protein and has around 4% calcium. According to the bag you can start that at 18 weeks but they are OK on the 15% until they start to lay. A lot of people wait until they see the first egg to switch or just offer oyster shell on the side until the bag of feed runs out.
The recommendations on the bag are for commercial laying flocks of chickens. Our backyard flocks don't have to follow such a rigid schedule. You have a lot of leeway in how you feed them.
You can offer treats at any age, just make sure you don't overdo treats. The feed contains all they actually need so you don't want to feed them so much other stuff you upset that balance of nutrients, but many and probably most of us feed other things. As long as you don't get carried away it doesn't hurt them.