As for the tanks on a VM--under the barrel has to be a skinny one, like a 9oz. Most 12oz tanks are fat enough that the tank will hit the barrel and 20oz is right out.
If you run multiple tanks: whenever a "pin-valve" tank is screwed all the way in it will be "on" and start venting out the other ASA(s). You can (and I have!) try to screw the tanks in at the same time to minimize this; you lose some and risk blowing o-rings but it is possible. Easier to have an on-off tank for this, or an on-off ASA, check valve, or other way to keep from venting all your CO2 every time you play musical tanks.
Lastly...if you're not in a hurry...in 3-4 weeks I'm going to be putting together some of my parts VMs and probably putting a couplle on the market. I could be interested in trades if you're still looking.
Two wrongs is a u-turn. It takes THREE wrongs to make a right.