I believe this Librem has 32gb, 2666 DDR4, single stick. I’m not sure the brand without opening it up. I’m looking at Crucial or Samsung 32gb for the second slot.
Will there be any problems mixing brands?
Are both compatible?
I dunno, it isn’t like mixing disc brands, there is not a cylindrical geometry format to worry about.
I think if the frequency matches you’re good. If the frequencies don’t match, then the faster one operates at the slower speed.
There are CAS latencies to consider as well.
My guess: The computer will probably sort it out.
I would make sure that I am buying from a company where a return won’t be a problem. Just in case.
Obviously that is, no pun intended, crucial information. You need to know this and I would generally know this by opening it up.
If it’s already 2 x 16 GB then you will presumably want to replace both sticks - and so the problem (question) goes away.
If it’s currently 1 x 32 GB then the question arises.
You also need to know the type of RAM stick (SO-DIMM v. DIMM, nearly always the former in a laptop) and the power (e.g. LPDDR4 v. DDR4).
The Librem 14 product page says “2x SO-DIMM slots, up to 64GB, DDR4” which is not maximally helpful.
What I usually do is
- open the computer up
- get the part number of what RAM is currently there
- convert that back to the spec of RAM (i.e. search on internet)
- then buy another stick with the same spec (easy option is same part number but sometimes that is not possible)
Another avenue is … I think there are other topics in this forum where customers mention RAM that they have purchased and found to work. So that is another source of information about the required spec of RAM, and part numbers that work.
I think “mixing brands” will not in and of itself be a problem. But you still need to purchase something that is of the right spec.
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A lot of consumers are buying SSDs anyway, where the geometry is essentially meaningless, may be bogus, doesn’t really matter.
A further complication is that even with HDDs the geometry might be variable these days (whereas the original implementation assumed a fixed cylinder / head / sector geometry).
As you were. 
i only say that because with certain brands of software mirroring (a.k.a. Raid/1) you can buy two discs the same size, (say two 144GBs) if one brand is off, or if the model numbers are different, they won’t mirror. I only assume at this point it must be the geometry. (And I mean sometimes, because sometimes it works.)
Like you say, meaningless with SSDs.