When do I change their feed
What is the purpose of these chicks? Eggs, meat, show, bug patrol, pets, breeding, eye candy, or something else. Some things, like raising them for show, requires a fairly specialized diet. For some other things it is not that critical. What breed or type are they? The commercial egg-laying hybrids might do better under a different diet than a dual purpose chicken raised for laying. Are yours only going to eat that feed or are they going to get other treats or maybe forage for some of their food? I'd want to know a bit about them and their purpose before I made specific suggestions.
any brand recommendations?
When I first got my dogs I asked a vet this same question about dog food. His response was that as long as the nutritional analysis was OK brand did not matter.
Some people's own diet may only include gourmet meals while others are more "meat 'n potatoes" type of people. Some are vegan or vegetarian, some are not. So only eat organic or non-GMO foods, others don't restrict their diet like that. Some enjoy spending a lot of money on their chickens feed since this shows how much they love them while others are looking to be a bit more economical. I don't know where you fall on this spectrum. That might influence what brands you want to get.
I'm not sure if the percentage makes a difference but I was running low on the 20% one and I could only find the 19%.
Some people seem to feel you are being abusive and cruel to your chickens if you don't feed them a fairly high protein diet. Mine do quite well on a lower protein feed. For some purposes a higher protein diet is called for. The breed or type can make a difference too. For most of us there is no significant difference between 19% or 20%.
when can I introduce veggies and stuff.
A broody hen raising chicks where they can get to living matter starts very shortly after they leave the nest. For most veggies and stuff they need grit to grind the stuff up in their gizzard. It is fairly typical for my broody hens to take their chicks to an area where they can peck at the ground and get grit in their system before taking them to an area where they can eat vegetation. You could have introduced veggies and stuff earlier if they have grit.
When chicks eat vegetation that is growing, the plant is rooted to the ground. That way, when they take a bite they break off a small piece. If what you are giving them is in long strands, like a long piece of grass 3 to 5 inches long, it can get twisted up in their crop and get stuck there. Not good. Keep that in mind to decide whether something needs to be chopped before you offer it to them.
At that age they do not need a lot of calcium. They need some for bone growth and general body maintenance. When they start laying they need a lot more calcium. Many of us offer supplemental calcium in the side and let them decide how much they want to eat. Most chickens at any age are pretty good at deciding how much they need.
They can get calcium from many different sources like their feed, some of the treats you may give them, from some of the plants they may eat if they forage, from bugs or critters they may eat if they get a chance, or even from the rocks and pebbles in your soil if one of your native rocks is limestone or they have access to limestone. Some flocks go through supplemental calcium really quickly. That means they are not getting much calcium from other sources. Sometimes they hardly ever touch the supplemental calcium. That means they are getting enough calcium from other sources. Instead of worrying about how fast your supplemental calcium is disappearing pay attention to your eggshells. If the eggshells are OK they are getting enough calcium from some source. If all of the eggshells are consistently thin you may have a calcium problem.
Good luck!