I plan on writing a piece for strings with some of each instrument in standard tuning and others tuned down to a few other tunings, and I wanted to check with string players if there will be any problems with using these tunings.

All tunings are down and the lowest are:

Violins - D-A-E-C

Violas and Cellos an octave apart - G-D-A-E

Basses - C-E-A-D*

*Or if using five-string basses - C-D-G-C-E

So are these tunings usable or will the strings be too slack or muddy sounding? (I'm thinking the bass C-E-A-D might be but would like to know for certain)

I'll add the retuned instruments are mainly for reaching certain natural harmonics and supporting open string drones under the regular tuned instruments so they won't be playing anything overly difficult.

Also, what are more common in the UK, basses with an extension for the E string or 5-string basses?

Any help would be great.