If you have thick wire, it is OK to use the molex connector.
Still have the issue with the limit of the PS on whatever +12v bus powers the "peripheral" connectors, and on an S7 you ARE talking about
(1) Appx 400 watts or well over 30 AMPS per hash board, works out to about 13 AMPS per PCI-E if you're using all 3 of them per Bitmain specs.
(2) limit of DO NOT POWER A SPECIFIC HASH BOARD FROM MORE THAN ONE PS.
The issue isn't JUST the molex connector.
In fact, it's more about limits of the Power Supply in this sort of case, though pulling over 6 amps through a peripheral-type Molex is marginal for reliability.
It's not smart to assume the current draw will work out exactly equal, parallelling connectors tends to have a little imbalance on the contact resistances and makes the power draw somewhat uneven and makes those adapters even more marginal (to be fair, the imbalance would probably be on the order of 7 vs. 6 amps per connector *IF* both are making a good solid connection - but if one is NOT making a good solid connection, FRIED connector and probably fried S7 hashboard).
PCI-E 6-pin connector - up to 13 Amps per each wire (3 12V wires per connector) = theoretically up to 39 Amps but ATX spec limits PCIe-PEG connectors to 75 W (6.25 Amps).
You have to derate the connectors used in a PCI-E connector by quite a bit.
Molex only specs them for 8 AMPs each max = 24 amps per connector = 288 watts in a PCI-E 6-pin connector, due to heat retention by the connector itself.
Even though the pins THEMSELVES are rated for 13 amps max that's a FREE AIR rating.
(Yes, I read ALL of those specs a while back - it's where Spondoolies came up with it's 288 watt max limit per connector in the SP20 web interface).