Yellow Leghorns? Not aware of Leghorns that are yellow. Generally, a bird with red earlobes will lay brown eggs and a bird with white earlobes will lay white eggs.
Here is the link to the breed standard from the Australorp Club of Australia. The weights are in kilograms, so you'll need to do the math for the conversion into pounds.
http://www.australorps.com/4.html
You can obtain a copy of the American Standards of Perfection from the American Poultry...
Don't wait past 6 months, the meat will get tougher with age. If your Australorps are hatchery sourced, they will be much smaller than Australorps that breed standard. And don't expect a dual purpose breed to look like a bird from the grocery store, once processed.
They'll be laying 'soon.' But how soon, soon will be is something nobody can answer without being able to see the future. Could be any time really, tomorrow or 3 more weeks.
I wouldn't eat them. You have no idea when they were laid, and in that heat, they are probably bad. Why risk it? You'll be up to your eyeballs in eggs in few more weeks anyways.
I did notice that one of your Leghorns has some black flecks. She's not pure Leghorn. She's likely a California White, which is a Leghorn crossed with a Production Black. She will out perform your pure Leghorns in terms of production.
Your Delaware is a rooster. See how the Leghorns have differently shape feathers near the base of the tail. The rooster has long, thin feathers that drape down. The pullets have broad rounded feathers.
I have a few that didn't start until 24 weeks. Roosters at that age would be obvious. Males have long, thin, pointed feathers on their back, near the base of the tail and "Fabio" style feathers around their necks. A 23 week old cockerel will look very different from a pullet of the same age.
It can be difficult to judge whether a chicken is too fat just by looking at it. A bird fed a balanced diet, and has lots of room to scratch and run around should be fine. If you have a bird that isn't very active compared to the rest, is a sporadic layer, and/or has trouble breathing you may...
They can get too fat, but it's not likely in a young, active bird. Buff Orpingtons are a large breed, and have very fluffy feathering, making the look even larger.
Soft shelled eggs are pretty common in new layers for the first few weeks/months, even when they have calcium available. Some hens will lay soft shelled eggs every now and then throughout their lives. Do you have crushed oyster shell available?