It all depends on detail and familiarity with subject matter.
If your using 20mm industrial buildings they're going to be more forgiving than domestic and retail. The main detail that's going to cause you an issue if your aiming for multi scale is Brick work. Be it Mdf kits or pre printed card buildings. It's the main detail that either needs covering or avoiding in the first place.
I personally rescale 'n' gauge and 'HO' for 28mm . I don't tend to then use them for 20mm .As I raise them up by roughly 10mm so the top of the doors are more in line with 28mm. ( I also but not always adjust the width of the doors too.)
You can get away without changing doors on industrial buildings as they tend to be larger in 20mm compared to domestic doors in 20mm. So they look more like a normal door in 28mm.
I've included a couple of old pics my stuff . The table has a mix of scratch built ,N & HO gauge conversions and 28mm mdf.( Sarrisa 28mm signal box.)The doors in the second pic are not rescaled on the 20mm conversion ( with exception of one single width door.)