In principle you can use the same cable and connectors for SDI and composite video.
The SDI cable must of necessity use proper 75 Ω BNC connectors because the digital signal bandwidth extends into the gigahertz region.
However, the BNC connectors fitted to video equipment are often 50 Ω even though the internal terminating impedance is 75 Ω over the video bandwidth. While this has no significant effect on the low-bandwidth composite-video signal, there may be a mechanical problem in trying to mate a 75 Ω plug with a 50 Ω socket.
50 Ω and 75 Ω BNC connectors may differ slightly in their physical dimensions, so that a thinner 75 Ω pin on the SDI cable may not make proper contact with a 50 Ω socket.