Great to read your girls have settled in so well
In answer to your question on when to change from growers to layers...... I've bought hens before who were 15 weeks old and they've gone straight onto layers pellets or mash as that's what the rest of my flock eat..... One hen I bought at 16 weeks laid her first egg the following week! The others all seem to come into lay around the 20 week mark......
Continue to enjoy your girls - best stress buster I've found so far and that's coming from a working Mum with 5 kids under 16!