Every note in the chromatic scale is a possible choice
Chord tones (notes from the chord) will sound the strongest and most "inside"
Non chord scale tones will have a less strong sound, they should usually be accented less
Non scale tones have a very "out" sound, and can be used, but are best used as passing tones, played on the weak beats.
My approach is usually to think more about arpeggios than scales, and fill in arpeggios with scale notes. Also, thinking about each chord seperately can lead to choppy, cut up sounding soloing. The goal (and this is very hard, I'd like to think I'm here yet, but I'm not) is to weave lines that work through the chords, to develop musical ideas that doesn't start and stop at each bar line. This can be done by connecting chord tones. In ii V I progressions, it sounds good if you go from the seventh of one chord to the third of the next (from the seventh of the ii to the third of the V, from the seventh of the V to the third of the I, and land on the third at the beginning of the bar lines.)
Sorry if this is a lot of info to digest at once, but if you stick with this, then you'll have nicer, more musical sounding results than
mixo goes with 7 chords
locrian goes with half dim chords
ionian goes with maj chords
etc.
