I'll start by saying that I have read a bunch of threads including the main thread discussing the Ecco conversions.  However, something that I couldn't find a clear, definitive answer to was related to choosing a new mounting system (Keymo, Xeno, Plan B, etc).  I have a few different 51t AAC suppressors and I'm wondering whether I could get them converted to standard 1.375x24 threads and then figure out the specific mounts I want after I get them back (maybe even trying different ones to determine which I like best).  It seems like you can buy the mounts directly from the manufacturers, but I keep seeing that people are trying to decide what mount before sending them off to Ecco and it seems like you could have them convert to the standard threading and just deal with adding a mount on your own.  Perhaps there is a legal issue with owning silencer parts?

Based on my (limited) understanding, it's more of ensuring the fit with the new mount module/system is good versus anything else.

My SDN6 is headed to ECCO right now.  

I don't think there's any issue with having multiple mount modules.

ECCO just threads the end of the silencer to the omega/bravo pattern thread pitch (1.375x24). It's up to you on what adapter you want to use and when to get it. Most people were getting their mounts before so they could start getting muzzle devices. There was also a shortage of omega keymo mounts, so people were snatching them up as fast as possible, even before sending their can off.

ECCO will cut off the AAC latch, add a thread conversion piece to accept 1.375x24. ECCO prefers to have your mount to confirm thread compatibility. I believe ECCO finds 1 out of 10 mounts may not meet ECCO tolerances.

Mounting system is your choice. Price and weight seem to be the determining factors in mount selection. An example is DeadAir Keymo at $125 vs Yankee Hill QD at $80. Really your choice on mounting system.
I would recommend getting in the cue.

Not my tolerances.  Lol.  ASME & ANSI thread specs.  The max pitch diameter for 1.375-24 3A is 1.3479".  I use a certified plug gauge that's 1.3479 +.0003, then cut threads a tad deeper to end up with a 1.350-1.352 pitch diameter, comfortably in the 1.3479-1.3536 2B range.  

Except for DA adapters, we rarely see fitment issues, so those are the ones that really need to be included.   It's generally an issue with surface treatment on them, which I can take care of in a couple minutes with abrasives & needle files.  Five out of six in the batch I just finished needed that.