It looks like your post has been hanging out here for a while, and I’m sorry we overlooked your question!
You may have already run across this post on the forums, and if not, I’d recommend it to you as an excellent step by step.
You absolutely can use high-temp concrete mix for the entire slab. Or not at all. There’s no actual need to use high-temp concrete. As long as you’re following the general drift of the instructions, modified for doing a monolithic slab, heat never really affects the hearth slab itself. It’s an insulated slab for that reason. The standard bricks that form the frame for the firebrick hearth absorb heat, and the layers of firebrick/sand/insulative material that form the hearth are designed to make a thermal break between your hearth and the slab.
The important thing is to have the high-strength concrete (4000 to 5000 psi) throughout the slab, along with rebar to diffuse weight stress.
What you’re proposing might be okay, but I couldn’t speak to its performance. Also, you’ll be “bricking” yourself in if you fasten your firebrick in place with thinset. What happens if a firebrick cracks? How do you replace it? Ideally your brick are resting on a sand layer that allows easy leveling AND easy replacement.
Knowing that some time has passed since you posted this, I am curious what you decided.