I want to mix these two chemicals to create a solution for a molecular
biology application in which the individual properties of each
chemical is desired (penetration of microbial cells, and stabilization
of RNA in those cells).
My question is simple: will the two named chemicals
interact reactively (either dangerously or such that their properties are
altered)? Links to scholarly or commercial sources desirable.

alright, since none of the google geniuses have posted on this I think
I'll field a WAG(wild arse guess).

OK, so if you mixed these things together like 15 days ago and the
solution isn't turning green and funny colors you might as well try a
bit of it to extract your RNA.  If it don't work, well then you know
the answer.

apriori, I would expect these two things to be pretty happy together,
but DMSO is a gnarly solvent that often discolors over time just from
sitting around in the bottle with a little help from impurities in the
air. Guanidine thiocyanate, might reduce, oxidize or otherwise do
chemistry we don't know about after just lying around.  If there's
water in there, then you got the RNAse's to worry about not to mention
other little green eyed gobblins like mycospore and other stuff.   So
the real question is HOW LONG will it be stable?  I dunno man.

In short though, if you're planning on selling this to some po schlep
who's buying "sockeye's RNA prep" kit well, $4.50 isn't really good
liability now, is it?

In principle, DMSO can be an oxidant, while thiocyanate could be a
reducing reagent.  But since the reduction of DMSO (in the absence of
light) would probably have to occur by O-atom transfer, and the
oxidation of thiocyanate is almost always a 1-e electron transfer,
those two should be compatible.  Keep such mixtures away from light
though.