Second, I don't know how they sex the chicks. There is a thread on here some where about sexing the chicks. There is a chance that even though they are all supposed to be pullets, there could be a roo in there. I had purchased all pullets from the local feed store and one turned out to be a roo. I don't think that it happens very often, but it does happen occasionally.
Third, yes they do chest butt. (I learned that off of here.) In my opinion, when one or two intervene, then they are starting to show their pecking order, who is going to be boss.....lol
If I find the thread about how to sex the chick, then I will post it back here.
In RIR the girls get tails and wings sooner than the boys and the boys get combs and wattles, sooner than the girls. And, the sexing is only about 90% accurate. If you bought 25 pullets, 2 or 3 will turn out to be roos. I bought 6 pullets and 6 straight-run RIRs a few weeks ago. I have two little roos out of the dozen. Each one had no tail or wings feathers while the girls had both. Now the roos have bigger combs.
I had to run outside and take a look, yes the ones with the longest tails have no comb or wattle, the one with the biggest comb and wattle has a medium size tail compared to the other 4.
I'm just a city boy from Milwaukee transplanted to Kentucky who likes eggs. I like the site and will help out where ever possible, my expertise is 30 years of automation electronics and heating/cooling systems.