I know the hens that laid the majority of the eggs are a mix of Brown Shaver, Brown Shaver crosses
Brown Shavers are a commercial sex linked egg-laying hybrid. As such, they have relatively small bodies (about leghorn size). I don't know what those Brown Shaver crosses are crossed with, other Brown Shavers or maybe some other breed or type.
and rescued "meat chickens".
This has me confused. Don't go getting the big head, I'm easily confused. Confusing me is not much of an accomplishment. Typically "meat chickens" are Cornish Cross, designed and fed to be butchered at 6 to 8 weeks of age. How would you get rescue chickens from these? Occasionally a few can live long enough to get eggs but most die if they are not butchered at the right age.
Perhaps these rescues are from the parent flock that lays the eggs that hatch out the meat birds that get butchered. Some hens have to lay those eggs. I could see rescues form that flock.
Perhaps you are talking about Rangers. These are designed to be harvested at closer to 12 weeks but are typically good foragers and can live to lay eggs. I'm not sure how things work in New Zealand, perhaps you have something else.
The way I read your post it sounds like some of the mothers are Brown Shavers, some Brown Shaver Crosses, and some Meat Chickens. The photos are not real clear, looks like you took them through plastic, but except for the small one they all look like they had at least one white parent. That is not going to happen with a Brown Shaver hen and Rhode Island Red rooster.
I do not weigh my chicks so I do not know what a two-week-old chick should weigh. Finding reliable information online is proving to be a challenge. I could not ascertain breed or type but what little information I found implies that your weights of around 130 grams are on the low side. I don't think those are meat bird weights, they may be more in line with the smaller commercial hybrid layer weights. The other one may be a bantam or a runt. Maybe someone else has reliable information on what a 2-week-old should weigh for various breeds/types.
Can you find out anything more about the parentage of those chicks? That's not always easy. What meat birds do they have? What rooster was used, Brown Shavers and a RIR are not the parents of those mostly white chicks. Other than the small chick, were they all from the meat birds?
I don't think I answered any of your questions, probably left you more confused than you were at the start. I feel that way.
