Slash chords are a completely accepted form of chord-symbol, they have been around a long while now. They are not a cheat designed for guitarists, they were developed originally by pianists. Their primary use is to point out stepwise or chromatic movement in the bass between chords.
Chuck Sher's New Real Book makes extensive use of slash chords. They are part of the standard leadsheet vocabulary at this point.
Polychord symbols are not in common use. In fact, the only place I can remember offhand as having had polychords notated is in the first place I ever saw them -- Ted Greene's very informative but VERY NONSTANDARD Chord Chemistry. (Greene has his own little symbol world. He's a very fine guitar player and instructor by all accounts, but he's not helping things IMHO when he muddies the water with his idiosyncratic symbology) If someone does actually use a polychord symbol on a lead sheet they will no doubt make it clear that they are using polychords.
In my experience polychord thinking is mostly an analytical tool for looking at chords. Polychords are not uncommon, they are common as dirt... Any chord built in thirds and larger than a triad will be a polychord of some kind.
To take an extreme example, let's take a large chord... How about a G13 chord (with the 11 voiced, as is not typically done): GBDFACE
That's all the notes of the C major scale, so forget triads, forget polychords, that is a polywollydoodleallday chord, -- it is all the chords possible in the key of C major.
In a less academic vein:
Every seventh chord (dominant, major or minor) is a pair of interlocked triads, every ninth chord (dominant major or minor) is three overlapping triads (this is quite useful to know for substitutions and arpeggio improvising. Indeed there is a whole improvisation techniqe called triadic improvising which makes copious use of interlocked triads... this is mostly a newfangled way of applying the old concept of chord substitution IMO.
The entire notes-stacked-in-thirds concept of major scale harmony takes triads and overlaps them like the olympic rings.
PHP Code:
CEG
EGB
GBD
BDF
DFA
FAC
ACE
CEG
And, if you haven't yet, memorize: CEGBDFACEG <--- it's all diatonic chords in thirds, in all keys (just drop in your signature's sharps or flats as needed)
And, why not, for Bill Evans quartal harmony there's:
CFBEADG <--- play this as a chord at the seventh fret, it's a nice and easy symmetrical fingering, that shows there is some logic to the kink in standard tuning. (LH tap the G at the end.)
Doug.