You can download streams from YouTube because it has had special handling by JRiver. You can't actually record YouTube videos, just watch them, save a link to them, or download them.
You can do that for some other streaming sources, but it is a bit hit and miss, and depends on how the source works, I think. Most streaming video sources played in the browser inside MC just play in the browser, and don't even use the MC Player. As a simple check, if MC's Audio Path doesn't show what audio is being played, then the video is just being played in the browser.
I don't have SlingTV (can't get it in Australia anyway), so I don't know if MC could download the video or not, or play it in MC's Player. I would think not for a commercial streaming service. But try it.
Note: If you try to open a video stream by clicking on it in a browser window inside MC, and there is no popup asking what you want to do with the stream, cancel that and then hold the Ctrl key while clicking. Then MC will offer the options to "Show the web page", "Play the file inside the player", "Add the link to my library (no download)", and "Download the file and add it to my library".
PS: Some have said you can add a stream URL to a Playlist, and then it can be played by playing the Playlist.
Disclaimer: I don't have any commercial video services, so don't have lots of experience in this except for YouTube and a couple of other sites. Therefore, I could be wrong!