ok well when to use them?...u can use them whenever u want but you do not want to go really crazy with it because like i have heard alot of gospel pianists who go to crazy with it and it sounds bad because it is like you are saying this is about me...but my suggestion is to use them at the end of a line in the song...a quick example...Lord You Are Good...verse 1 lord you are good and your mercy endures forever---passing chords here...same goes for in worship songs..at the end of a line but not all the time!...I also suggest play the verse the first time normal (without passing chords) and then when you go to the second verse or repeat the verse add passing chords
And to answer your question about which passing chords to use in every key....IT DOESNT MATTER!!!!!...lol but Of course let your ear guide you...but if u havent made up any passing chords on your own use the progressions and incorporate those into which ever song you are playing...and maybe i didnt answer your question and you just wanted passing chords...here are some
To go to a minor chord...for this example i will use an F#m
Do this
G/Bb-C#-F#
and land on your F#m chord
here is another....when going to another minor...for this example I will use Cm
Do this
C-G-D/Eb-Bb-D-F
and land on your Cm chord
Another...when going to another minor....for this I will use Bm
Do this
F#-C#/Bb-D-E-A
and land on your Bm chord
Another I have said this in another thread but here goes...this time when going to a major chord (preferably Maj7 or Maj9)...I will use Gmaj9 for this one
Do this
D/C-E-G-B-D
A/C-Eb-Gb-B-D
and land on your Gmaj9 chord
ok here is something else use tritones and ditones(Jamal heartwell stuff)...i use alot of ditones...for example use this chord for your diminished chord in Ab
C-E/Bb-Eb-Ab
Hope that helped Be blessed..any other questions ask....