Is matching the tire (brand and tread) that critical.
no, not at all, it is done *all the time*, extremely common. the internet is ripe with black and white apocalyptic talk of tires though so you'll have to wade carefully to find the best fit for you. i'll try to help since i know how common this is and what and why you have options.
Replacing all 4 is wise advice for the masses who don't have time to focus on options, details, keeping track of maintenance, etc - and i am this way in other venues because i don't have time - it is a very wise one-size-fits-all anecdote to be sure. But there are other approaches that are fine and they are common place and offer an excellent fit for certain people.They won't be known by those that have never tried, seen, or done it once...much less hundreds of times, so you'll expect all of those folks to not buy into this at all since "suspicion often creates what it suspects".
i'm not recommending, nor have I ever done this, but it's just an example of how technically deprived many tire responses are. They aren't driven by technical aspects, they are driven by literature printed by Subaru - which is good advice as I've already said, but nonetheless some folks have the capacity to look beyond just what Mother Subaru says:
One *can* (be careful to do it right) install two new tires on one front and one rear opposite side tires because of the way the front, center and rear differentials work. This is done all the time by tire shops in Subaru rich areas that know this. Unless shops are well informing their customers or they are coming back for all future rotations though I think that's risky for a shop to do.
If you've even seen a Subaru stuck in the mud, snow, high centered, etc - you will notice that one front tire spins and the opposite side rear tire spins - the reason for this is closely linked to the previous paragraph about differentials.
Again - im' not suggesting this - i'm just suggesting that the one-size-fits all anecdotes that are blindly followed without experience are not practically well informed about how differentials work, why this tire stipulation is in place, and what is actually happening and what is actually possible.
There are options for folks who want to learn. Which is probably only a few percent of people...which brings us back to one-size-fits-all approaches being a very good idea....so tread carefully I agree. Follow the advice of someone that knows what they're talking about and don't sell or give the car to someone with wonky tires.
will I be okay for the next six months when I plan on replacing all 4 tires with new, and probably back to factory specs.
you will be fine indefinitely so long as the tires are matching in size (tire size and tread depth) and you rotate them properly.
this is not a big deal. you'd get a much broader and more technical advice on this question at USMB.