Yes, it is universal. Of course, a single cable if is correct cannot cause an explosion. Your seller obviously joking with you.
the only thing you need to pay attention to is the quality of the cable. Low quality can bring some problems on load.
Pinouts on different PSUs are not same, so seller might have a point. Even within one brads pinouts might differ from model to model.
yes, but it does not have to be the same manufacturer. If pinouts are different, he probably can't connect them, but he can use the adequate compatible cable from another manufacturer.
I still think that I'm not missed the essence of the OP's questions. We have a manufacturer who produces these cables and adapters for different standards which are used by all PSU manufacturers.
TheBeardedBaby explained what to pay attention to and it is not obligatory to use EVGA Molex or s-ata adapters, it can be OCZ for example.