Need advice

Storage tot- 21w x 14d x 13h
125 w heat lamp
With wire rabbit cage on the top to keep kids and pets out but maintaining an open top
6 chicks in one
8 chicks in another

Got some rabbit cages but on chick tried to escape through the wires so figure I'll use those in a couple weeks

The first thing I would suggest would be MUCH larger tubs, and a covering on them that will allow lots of air circulation.

Agreed, the bins will be outgrown pretty quickly, and you don't want to run into a problem with brooder boredom. It can lead to feather pecking and other bad habits. I have two in a 36x18 clear plastic bin and they are getting antsy at almost 3 weeks old. I think it's time to expand their space. You could use a kids swimming pool if you have plenty of bedding so they don't end up walking on the slick plastic floor. We have found very large cardboard boxes that we housed 8 chicks in, the boxes were almost 6 feet long and a little over 2 feet wide. They did great in those til they were fully feathered and ready to move into the coop.

I plan to move my 3 week olds out to the coop with the rest of the girls probably later this week. Right now I have Cornish Cross chicks in there waiting for their run to be completed. I created a pen of sorts using wire shelving and chicken wire to keep them separate from the bigger girls til they got used to each other. A week later and I just leave the side of the pen open so they can dash in there if they annoy a hen and they need to retreat but they seem to be doing fine. My other chicks will go in there and the process will be repeated acclimating them to the coop and the other chickens, and vice versa.

Chicks grow so fast, you could start out with a small brooder box, but, plan to expand pretty quickly. For an example, we picked up a buff orpington and a golden sex link and planned to get the black austrolorp chicks that IFA had on order for the next week and just put them all in together. Well, the austrolorps didn't hatch, so we ended up going with Jersey Giants instead but by the time they came in our other two were already so much bigger than the new ones that it wasn't safe to house them together. They are not quite two weeks apart, but, it made all the difference. In that two weeks the size difference was four times.