I would buy a bigger PSU than the graphics Card (120 Watt), to have extra reserve for power peaks, but yes, this can be used.
Connect it to the riser cable, the riser should power both, the card and the Helios board. No Need for second AC adapter, if it works like my Thundertek/PX.
Google if the Sonnet Echo Express can provide 75 Watt via PCI-E port, maybe you don't need the riser cable.
Understood but if I use two 60W power supplies at the same time would that work?