1. Is a Corsair 650 PSU enough to power six gtx 1660 not super not ti.
The power consumption of a 1660 is something like 75w to 90w, and you have the power of Motherboard, CPU etc
6 of them is not advised to run with only 650w, and even if you can run, you'll not have enough PCI-e to run
And another thing, with ATX PSUs it's better to run with some extra capacity, a 850w gold PSU could be good for you
2. Who is running a add2psu here are they reliable for running two PSU at the same time?
add2psu is simple enough to not worry about, it's just a starter for secondary PSU, it doesn't make any difference, even you can start manually a secondary PSU
3. Can I run an extra GPU in a m.2 slot on my motherboard ?
There's adapters to m2-PCI-e, I don't know which motherboard you have, some can run without any problem, some have limits