Well, there could be another reason why it was tough. At the start of the thread, the question started out like this: "So I have a flock of 4 month old RIR's and Light Brahma's. I'm only going to keep 4-6 of them as layers, so the rest will be broilers".
"the rest will be broilers"? wait a second, chickens that are bred solely for egg production don't make good meat, and "dual purpose" birds which are for eggs and also meat also have not-so-good meat. Chickens which were specifically bred for meat, which are called "broilers", have the best and most tender meat because they are bred for that.
So, taking a RIR chicken and telling yourself that you will eat it does not make it a "broiler". What makes a broiler a broiler is a specific series of selective breeding steps, this develops a variety of chicken to be fast growing and to have lots of tender meat on the skeleton. This type of bird is referred to as a "broiler", they are white with yellow skin and you'll notice that when raised with layer or dual purpose chicks they will grow 2 or even 3 times faster than the other chicks.
So, perhaps this chicken was tough because it was not a broiler, but rather a layer? Lots of laying chickens have really rubbishy meat, because they were bred to produce eggs, not to be eaten as meat.
Try buying some broiler chicks, and raise those. They will be ready in between 7 and 14 weeks, depending on their conditions and also on the variety you choose.