I am trying to clear up an ugly maintenance plan on SharePoint databases that happen to also be backed up by Data Protection Manager (which is setup to take snapshots at 15-minute intervals). The business would like to separately perform backups of the content using the built-in SQL tools. Will performing database backups, transaction log backups, and/or transaction log truncation within SQL Management Studio interfere with the DPM backups/restores?
You'll be absolutely fine doing this. DPM uses the VSS mechanism for performing backups, which does not interact with the native LSN-based backup mechanisms. The only thing to be wary of is that if you switch into the FULL recovery model, the first VSS-based full database backup will do the same as the first full native database backup - and properly switch the database into the FULL recovery model, where log backups are required to manage the size of the transaction log.
With DPM backups, if you're in the FULL recovery model, you must also take log backups otherwise the transaction log will grow forever. The alternative is to use the SIMPLE recovery model where log backups are not required and the transaction log will truncate on checkpoint.
So, you are absolutely safe with the plan you describe.
DPM works at the sector level. It keeps a map of which sectors (or clusters?) have been changed, and it backs up the changed sectors. Thus it doesn't know about what files have changed, just which sectors.
If you do a SQL backup you're not changing any sectors (unless of course you're backing up to a file on a partition protected by DPM) so the two won't interact beyond increased disk loading if both are running at once. If you truncate the logs then you are changing sectors and DPM will copy the changes. However I can't see that this would be any different from the normal changes in logs that happens all the time.
So I would say that you're perfectly safe to use both DPM and SQL backups. Would I swear on the bible that there is absolutely no scenario in which unfavorable interactions could occur? Well no, Windows is too damn complicated to be sure of anything, but I think it's very unlikely.
You should only take backups with a single platform unless you are very careful about how you set them up. If not, you won't be able to correctly restore the backups to the server.
Remember to keep in mind what your restore SLA is. If DPM is backing up on a nightly basis and you are not backing up the transaction logs throughout the day you will only be able to recover at best to the previous full backup. If you need finer grain restore than that remember to also back up your transaction log on a regular interval.