For P90 vest, your pretty much stuck with modular vest as a base and P90/UMP mag pouchs on molle.
It's now easyer to find P90 mag compatible pouchs. I had to have made on a custom job for me 4 years ago.
Forget about the stargate setup. They don't carry spare mags for read. When you see them swap mag, they take them out of the vest from under it, like if it was in an inside pocket... Not realistic or practical at all.
Some of my teamate use double or tripple AK mag pouchs to fit their P90 mags, but that setup is builky and it wobble a lot.
This is my setup. All custom made. Except for the green P90 mag pouchs.
On my black SWAT rig. (I'm on the left)