The three fields thing is really more just bad design on my part. Each RTMP provider has their own URL parameters.. Most seem to rely on just the playpath and channel name (hence the three boxes for server, playpath, and channel), but some also use extra parameters (like livestream apparently). I'll be modifying the design there so that each service can use their own custom set of parameter fields.
For the custom, It could just be one box where you can enter a full RTMP URL, such as "rtmp://server.com/channel/playpath[/appdata1][/appdata2][/...]", but I don't like the idea of that data being exposed to the stream, so I may leave it as is and add more info on usage in a tooltip after I get tooltips in.