M/S widening is inherently mono compatible.
But... what is it you're trying to do? That's probably the best question to answer first.
Do you mean to say there is more likelihood of problems in the mono mix if i'm using a combination of L/R panning, stereo delay and stereo widener plug ins?
I'm fairly new to mixing, i've read as much as i can, heard a lot of stuff that i don't understand or problematises what i thought i knew, and i'm trying to lay some basic ground rules/codes of practice for a nice wide (and hopefully not too messy) stereo mix.
I've got a lot of soft synth instruments in the mix, and i'm basically trying to test the water and see if there is something else i should know before going in there and trying to pull off a good stereo sound by:
1) Keeping kick, bass (lower end at least) mono
2) Widening mono soft synth sounds that are important in the mix with a simple stereo spread tool, whilst panning more decorative mono or stereo sounds left and right.
3) Leaving in built in stereo fx on certain soft synths like the Nexus2 that i like, but don't feel i can necessarily scrap and reproduce myself, and hoping that they won't cause problems in the mix.
I guess with judicious use of a stereo imager (i have a copy of Waves' S1) on tracks and the master bus (also have a copy of Izotope Ozone) i'll know if i'm doing something that stinks, whether i'm pushing it too far or whether the stereo needs boosting here and there. Again though, i'm kind of concerned as to whether, having got the mix sounding as good as i can, i can then go and start using a multi band stereo imager to widen out certain frequencies that may well already have stereo delay fx etc going on. As i said, because i'm new to this i feel i need be clearer on what i should and shouldn't be doing, so things don't get messy.
I won't be doubling any stereo tracks for Haas stereo movement (i'll probably use some stereo delay plugins, or double mono and put a chorus on one), so hopefully i should be getting any phase cancellation when i mono the L/R stereo mix.