As far as I know here is the stacking part:
All Mana Regen stacks up to 15, including same number. For example, if you have MR1+MR1+MR3+MR2+MR2=MR9 or 9 manas per tick. You can stack this up to 15.
You can stack that with chanter spells, C1-C5. What I found here is that the higher spell over writes the lower ones and group one over writes single ones. (last part I am not 100% sure but fairly certain). So, if you have Breeze or Clarity, KEI will over write it. I usually use C2 after a rez on my chanter because it takes very little mana and I can regen much faster for VoQ (C5) casting and C5 overwrites C2.
The chanter line of spells stacks with shammie Spirit line of spells. I am in heaven when I get both on me as a cleric because that mean very high mana regen.
The Mana Regen, chanter and shammie spells all stack with bard mana regen song. The highest bard song give more mana regen than any of the individual spells.
Lastly, I am very certain that AA regen stacks with all of the above as well. I was told that AA adds in to the Mana Regen 15 max but I am not 100% convinced. Even if it is, I don't have MR 15 yet so it doesn't matter atm.
If I have a group with my MR 10 (need to get that last 5), 1 from AA, chanter and shammie buffs and bard song, I can go through LDoN adventure with never having my mana below 80. It's very convenient. Even a hard adventures, I rarely have to stop for mana break.
Taushar