welcome to BYC and chickens!
Squatting does mean they are sexually mature, and they are deferring to you as a rooster. When my hens squat for me, I just ruffle their back, they shake it off and go about their business.
At that age, they should be laying ...moves can cause an interruption in laying for a week or so.If they were in a larger flock, you sometimes find more docile hens that are smaller because growing up, they didn't fight their way to feed. Or could be they are smaller just because of their individual growth/genetics.
Another possibility is that the guy was hatching batches, and the smaller are just a bit younger than the others.
And, it is possible for some hens (this is pretty infrequent) to just never lay. not likely but it does happen.
Enjoy your chickens and take the advice here with a grain of salt - use your common sense.
One piece of advice, try to check your hens out thoroughly about once a week. That means, while on the roost, pick them up and look them over. Check for mites/lice, look at the bum, feet - you just want to get used to normal conditions and to be able to spot problems early.
They are easier to handle on the roost. If you have a headlamp, you'll find it super easy to do it by yourself.
Good luck!