So Overload is an alternate cost. What happens if I can play a card without paying it's cost, am I allowed to use the Overload-mode? Specifically, what happens when I put Cyclonic Rift on an Isochron Scepter ?
You can only pay one alternative cost. The copy cast using Isochron Scepter uses an alternative cost, so you can't use Overload.
117.9a Only one alternative cost can be applied to any one spell as it’s being cast. The controller of the spell announces his or her intentions to pay that cost as described in rule 601.2b.
How to autocard (do this to specify a card in your posts): Type [c]Black Lotus[/c] to get Black Lotus . Type [c=Black Lotus]The Overpowered One[/c] to get The Overpowered One .