The answer is maybe. RCS is a set standard like SMS, however, your assumption that it is baked into every version of Android 8.0 is not really correct. The OEM actually needs to have a messaging app on the device that supports that specific message protocol. My S8 just got the update in the October security patch to support it. RIght now though it doesn't really matter since T-mobile is the only carrier supporting it besides Sprint and there are so few devices to support it. As things stand I believe you will need to download Googles messaging app(which you said you did) since it has the functionality baked in(you have to toggle it in the settings). Based off what I read on the forms one plus has not added the functionality themselves and has made no comment in regards to the one plus 6.
This is a link to one of the articles where the discussion was occurring: https://forums.oneplus.com/threads/when-will-oneplus-support-rcs.822510/
Every current Android phone can have Google/Android Messages which supports RCS. T-Mobile is choosing to only enable a few phones at a time for some reason that they haven't made clear. It could be that they are requiring something that is not standard. But as they are opaque as hell on the details of RCS beyond some non-specific self-service PR pieces, we are left to flail about on the subject.
I agree it's possible it'll work, but not something we can guarantee. As mentioned above, RCS is standard and more of a broad messaging technology. Advanced Messaging is T-Mobile's version derived from RCS. We aren't able to guarantee devices we don't sell will get any necessary updates needed for this feature to work or that it'll function like devices designed to work on our network will.
So you are saying that T-Mobile isn't implementing the standard, rather they have taken the standard and twisted it to make a proprietary version?
That would certainly help to gimp the standard by ensuring that T-mobile customers have an inconsistent and problematic experience texting anyone not on the T-Mobile network.
Couple modifications of your list from my current understand and limited experience of getting it (seemingly) work with two galaxy s8's:
-A Network that supports (T-Mobile should be good, However I have questions, and it seems it's the kink in the chain.)
- A Device that supports (Any Device with Android 8.0 or higher supports RCS, as it's built in the OS) AND IS EXPRESSLY ALLOWED BY T-MOBILE
- A Messaging App that supports (Android Messages) - ANDROID/GOOGLE MESSAGES MAY OR MAY NOT WORK, VARIED REPORTS AND T-MOBILE ONLY GIVES DESCRIPTION OF SETTINGS ON SAMSUNG MESSAGES
- Carrier Services installed (On Google Play) - T-MOBILE HASN'T SPECIFIED WHAT SOFTWARE NEEDS TO BE INSTALLED/ENABLED (THAT I HAVE FOUND) SO IT IS LIKELY JUST SPECULATION BEING STATED AS FACT
- WIFI Calling Enabled/set to preferred. - T-MOBILE HASN'T SPECIFIED WHY (THAT I HAVE FOUND) BUT THIS IS LISTED IN SOME OF THE PHONE INSTRUCTIONS BUT THEY HAVEN'T SPECIFIED IF YOUR WIFI HAS TO BE ON AND/OR CONNECTED FOR IT TO WORK.
Net is no, your phone won't work right now (and who knows if it ever will, T-Mobile hasn't said). There is speculation about whether you have to have a t-mobile locked phone or not but nothing from t-mobile. There are a lot of other things that they haven't specified from phones, to messaging apps, to settings, to impact (if any) on your data plan. On the Samsung Messenger I've read that it is supposed to give you a visual indication of whether you are connected to a SMS/MMS peer vs RCS but that wasn't the case in our testing. You had no idea other than things like (if the other person had it enabled) seeing if they were typing and large files going through at full resolution. Sending those same files to a MMS person meant that they simply didn't go through and you had no way of knowing.
No idea why they are not taking the small amount of time necessary to help clarify questions so their customers can have a good experience. If they were a software developer they would be getting ripped to shreds by their community for such **** poor documentation and support.