Crossbreeding with commercial chickens.

A few quick points.
There's a strong trend on purebreeder-oriented chook forums to dismiss anything tainted with commercial bloodlines.
Unfortunately in the case of layers this can be an over-reaction against the 'Frankenchicken' style of breeding common to meat birds.

Meat hybrids (those white monsters) get to 6 weeks of age and start to collapse under their own weight. Having raised them myself, I can say that's no exaggeration. Of course a percentage will reach sexual maturity and be able to breed, but in general they're not a good breeding prospect, and the girls are very prone to prolapse, egg yolk peritonitis, and other health issues. They're guaranteed to make the owner feel terrible for having put them in the backyard.

So anti-commercialism in relation to meat birds isn't snobbery.

But commercial layers have never been quite so badly formed. This is partly because they have to survive an entire year of laying (rather than 6 weeks of putting on meat) and partly because laying itself is generally a good test of health. What's the first thing to go down when a hen is poorly? Laying ability. It stands to reason that, with the exception of vaccination (another issue I'll discuss later), these birds are quite likely to be fairly rugged.

For these reasons, despite a reputation ISA browns have for early reproductive failure, I think it's okay to attempt to crossbreed using them as a starter. They may be aggressive with other birds (they have a tendency to pick at one another), but crossing them to a more placid breed tends to reduce that to insignificant levels. Certainly outcrossing to a bird with good reproductive health (even if lower egg numbers) should help dilute any of those genetic predispositions.

So why be a snob if you can't find purebreds that lay well enough to make a backyard utility flock?

For that matter, you could probably breed out the health problems of the meat birds... But to do that, you have to get over the 'ugh' factor... Not an easy thing to do when the very sight of the poor things makes you feel fowl!