Yes you can have 6.3 Event Collector instances which forward to 6.2. In the configuration of EC you can select an output group for it to forward to. The receiving indexers do not have to be 6.3.
As to the UF, it is not supported today, though it may work. Only HWF is supported from a forwarder perspective.
I'm not seeing anything that says that the functionality does not exist on [universal] forwarders but haven't tried. I'd say give it a try and see? You can run a curl command against it to see if it catches your http request.