I want to install Windows Server 2016 and SQL 2016 which looks like is only supported on ITCM 14.0 SP2.
Can I install ITCM 14.0 SP2 on a new downstream DM server (and the respective SS and Agents) when the ES and other DM servers are running 14.0 SP1?
Yes but you should do a top down upgrade. In other words do the em first then your new DM. Then eventually upgrade the rest.
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I know that CA always recommends a top-down upgrade. I am hoping that there weren't any (or many) database upgrades between SP1 & SP2 to require a top-down upgrade. I am hoping I can go ahead and upgrade my DM and it's respective SS and Agents now without any issues (or "major" issues, anyway) while I wait for my slower counter-parts in charge of the ES to upgrade to SP2 later.
that would i not do. SP2 have database changes; we have done this in a past version and we had massively problems during update of the EM, that only the support can solve.