Can Active Guard Reserve (AGR) and TPU (Reservists) be senior rated against each other?

SSG(P) (Join to see), you are being rated against your contemporaries for potential by your Senior Rater. Your rater only rates your performance. What your rating chain is doing is appropriate.

NCOs aren't really part-time in the truest sense of the word. A TPU SSG/SFC must accomplish all of the requirements for their Leadership position in 2 days. This requires pre-planning and work outside of UTAs, including those pesky conference calls. They must maintain contact with their assigned Soldiers, many who really don't want to be bothered during the month for the BN/BDEs' last minute takers. This is on top of self-development tasks. TPU Soldiers, especially SSG and SFCs are notorious for horrible work-life balance, thats why we have a leadership gap at that grade. They are choosing to grab 20 and go.

If you have true "part-time NCOs", you have a marginal 1SG/CDR. There is a happy balance between giving time away for free and having a TPU NCO working constantly in a reserve unit.

As you indicated to CPT Lawrence Cable, your rating scheme is appropriate. As a Plans NCOIC you should be rated by the Plans Officer (CPT) and Senior rated by the S-3 (MAJ) who also senior rates all the SSGs in the S3. My guess is that you have maybe 3 other SSG AGRs in your BN HQ (unless you count the subordinate units), none of which are senior rated by the S3 OIC.

Even if you could manage to be compared against the other SSG AGRs, would you really want to have your potential for positions of greater responsibility and future operational assignments compared to supply, medical or personnel folks? A TPU SSG in FUOPs is likely to never be an AGR OPS NCO, so I would agree with LTC (Join to see), if you are not marginal compared to your peers, you come out ahead every time. Unless you are suggesting that a TPU SSG is not your peer, in which case this would be a different conversation.

Who's chain of command do you belong? AGR's in my unit got EER's from either the First Sergeant or myself as Rater or Senior rater. If you are occupying a line item position in a line unit, you should be rated by you COC in the unit, whether you are full time or not.
Since your position is generally support for the unit and the part timers, how those soldiers view your performance is just as important as whether the full time staff thinks your wonderful or not.

Absolutely. You are being rated and senior rated not AC, AGR, TPU, IMA, etc against each other but against all of your peers of the same grade for the duration of that rater and senior rater’s career. Unlike other services that rate you by year our profiles are built over our careers. Some raters and senior raters are biased, some pool though it’s against the regulations (yes I said it), but others do the right thing and are fair. Unfortunately not everyone who deserves the top box will receive it based on population, quality of peers, or poor profile management.

I see your argument and I have seem AGRs argue that they should be rated by all AGRs as TPUs do not know what they do on a daily basis. It’s unrealistic as soon as you get away from a company or go up in rank a little your chain would be at another unit and unable to see you on a daily basis as well. That is why you do your support form. It’s supposed to be a conversation with rated, rater, and senior rater. Does that always happen? No. But write your support form as you know best of what you did. You cannot control what your peers do. You can only control what you do and quality of work.

Do your best, work with your chain of command, control what you can.