Except for the the contrib & non-free branches of the Debian repo, PureOS & Debian should be basically identical except for a few specific PureOS/Librem packages. Debian makes the packages & the Purism team adds them to their repo afterwards, usually within a few hours, so mixing the two shouldn’t normally hurt anything.
That being said though I should point out that I installed my system from a Debian-testing cd so I could have better control over my base install. I also use apt-pinning to only enable a few packages from PureOS & from Debian-unstable.