I am interested in this round for youth deer hunting with very little recoil. To my understanding, I can use my 5.56 AR lower and 5.56 magazines to mate with a 6.5 Grendel upper? What are some recommended Grendel uppers that would be well suited for an accurate hunting rifle? I would Prefer no longer than 18" length.

I don't have a Grendel but I think you can stuff a few rounds in a regular 5.56 mag and it will feed. Any more than just a few you are going to want to get a 6.5 Grendel magazine. Otherwise yes, you can use a 5.56 lower and mate it to a 6.5 Grendel upper.

I've heard Alexander Arms and Odin Works make pretty good 6.5 uppers.

I have no experience with one but been coveting one for a while and looking into it. Here's what little I know.

The mags are different. As mentioned, I've also heard a few will work in a standard mag but u really need a dedicated 6.5/7.62x39 mag.
Radical arms apparently makes some pretty good uppers too in 6.5 Grendel.
The dirt cheapest way I found, if u or someone u Ono can build an upper, is an Anderson barrel and a bolt. U can use all the other upper parts of a 5.56. Just be aware that most 6.5 barrels have a thread pattern for ar10/308 at the muzzle.

Hardened Arms also has uppers.

You will likely have feed issues with a standard magazine. E-Lander mags from Alexander Arms will probably be the best magazines for 6.5. It's what I use on mine.