Each pin of a 4 molex is rated by the spec to handle 11 amps which amounts to 132 watts on the 12 volt pin. This idea it can only handle 5 amps is misinformation that is at odds with the official spec.
You can use a single molex to 6 pin no problem, or if you really have to use sata then you need 2 sata to 1 6 pin.
You are right. But read fine print before you connect 6pin to single molex.
http://www.playtool.com/pages/psuconnectors/connectors.html#peripheralI would play it safe and use two molex->one 6pin.
Same goes for sata to 6 pin.
What fine print? Some guy makes a website and says it's specced to handle 13 amps but suggests you "play it safe" and don't use more than 5 amps. His reason being nothing to do with the connector itself but the wire gauge typically used. 18 AWG is usually used for both sata and molex but the connectors themselves are only rated for 4.5 and 11 (or 13 if you believe him) amps respectively. 18 AWG can handle 10 amps or more over a short distance, but the longer the run, the greater the voltage drop. Typically it should be fine to run 2 risers or GPUs on 1 string of molex but if possible it's always better to have a single (especially for GPUs).
So there's really 2 issues involved here. Firstly the connectors.. Can a molex on it's own handle the maximum current that a 6 pin is specced for? YES. Can a sata? NO.
Secondly wire gauges.. 18 AWG is ok to handle up to 12 amps in the typical wire lengths found in a computer case. More than that is risking it. This is why you shouldn't really use more than 2 connectors per string. 1 connector per string is better but often not possible. What you should never do, is power a connection that might draw up to 6 amps (e.g. pcie 6 pin) through a single sata connector. It will usually work but it will be operating well in excess of what it is designed for.