I would get a native seed mix from a reputable nursery, combine it with the wheat straw or annual rye. Your contractor can hydro-seed with the native mix.

And if the slopes are real steep I would cover them with an erosion control blanket, which holds the seed and the soil in place until the vegetation gets established. The cost of the blanket is considerably cheaper than regrading and reseeding.

Another option is to use a plant that will not make it through winter. Up here in PA, I could seed oats in late summer, get a nice thick growth of oats, then they would die before setting seed, leaving a network of roots in place to hold the soil. A little research should reveal if oats are winter hardy in your area, and if they aren't then you might try them, probably combined with straw or hydroseed.

Don't fertilize the soil is you plan to grow natives. Fertilizer will benefit weeds much more than natives.

If you plan to seed natives, you could use short-lived plants like Clasping Coneflower, Black-eyed Susan, Monarda punctata, and Standing Cypress (Ipomopsis rubra) all of which will reseed for a few years, but eventually give way to natives from the surrounding area. I definitely wouldn't buy a wildflower mix except from a very reputable source. many so-called wildflower mixes include lots of non-native plants that many native plant enthusiasts would consider weeds.

Finally, I'll say that I think the plan of spraying straw, allowing leaves to fall, and letting nature take its course is a good plan. You'll probably have to remove some weeds in the next couple of years, but as site ages you should see fewer and fewer weeds and more native. If you have spots where the soil washes out repeatedly, you can stabilize those spots with an erosion control blanket, but most of the slope should be fine.