And if you don't medicate your birds on the basis of guesswork as to what, if anything, is wrong with them, their natural body chemistry and immune systems will be able to continue to function and develop properly. Medication has downsides.
that's a bit outdated.
The following (freely available) article is a review of current thinking on methods that avoid prophylactic and scattergun use of meds on poultry, to prevent the further development of antibiotic resistance among bugs. Probiotics are among them...
They look like normal poop to me, and it sounds as if you are doing a fantastic job as a new chicken keeper :) They are growing fast at 12 weeks so there may be more cell turnover (including intestinal lining) now than might be expected when they are mature. Relax, and enjoy them. There'll be...