The reason for butchering young birds for frying or broiling isn't how much fat they have, it's how tender they are. Younger birds are more tender. If you fry an older bird it may be too tough to eat. A fried, 6-month old rooster makes a dandy chew toy for the dog. They are wonderful cooked in the crock pot, though. Cook them til the meat falls off the bones. Yum!
The amount of fat on the bird, has more to do with breed, diet, and exercise level than age.
I have older mixed-breed and dual purpose breed roos in my freezer, with almost no fat at all on them, and have butchered broilers at 8 weeks old that had thick pads of fat.
While you're learning all the ins and outs of raising chickens, yes, the best thing to do is start with some layers, BO's or whatever you like the best. Get used to them, and set up some broiler pens, there are lots of posts showing pics of movable pens. I'd advise those for your broiler chicks, so you can feed them the correct feed, while feeding your layers their normal layer feed, and move them to clean ground as often as needed, so you don't have to muck out a nasty pen every other day. You might want to try the color rangers, they seem to have fewer health issues than the Cornish X's. If you butcher at 8-10 weeks you'll have big, fat, juicy, tasty birds. If I have that butcher age wrong for color rangers, I'm sure somebody will jump in and correct me.
McMurray requires a minimum number of chickens, but they can be any number of whatever breeds you want. There are separate minimum orders for separate species, so if you want ducks, you must order X # of ducks, but you can mix up the breeds, for example, 5 Khaki Campbell, 5 Pekin, 5 mallards, and so on, until you reach or exceed the minimum number. Same with chickens, guineas, geese, etc. You can mix breeds, but not species.
McMurray's a bit pricier than some of the other hatcheries, but you can find plenty of posts about the quality of the birds you get from the different ones, and who has the best prices. You may want to look at who's closest to you, to minimize travel time for the chicks, unless you find a local source where you can pick them up yourself.
Good luck, I hope it all works out well for you!