How to start Internet Recovery Mode of El Capitan with multiple OS installed?
3 Solutions, or more likely workarounds
(Found them myself, but thanks to Thorbjørn Ravn Andersen anyways!)

To reiterate and clarify: I want to start in recovery mode of El Capitan on a MacBook Pro 2011 which has said OS as a partition and on another the ("factory set") Snow Leopard. Since El Capitan had - for an for me unknown reason?! - no recovery partition installed, my actual goal was to start internet recovery mode of El Capitan. This did not work since whenever I pressed [Cmd + R] at the chime, it started into recovery mode of Snow Leopard, which doesn't contain the wanted terminal command "csrutil" yet.

I went with option E)2. (above) and installed OS El Capitan on an empty external disk. After rebooting and holding the [Opt key] at the chime, I was able to choose the recovery partition of the newly installed system on the external drive, and afterwards disable SIP.

Thorbjørn Ravn Andersens solution will probably work too: A bootable USB stick - that's essentially the same as I did, but might be faster (?)... A third solution is probably (E)2.) reinstalling El Capitan to get the recovery partition (again?).

But even though those solutions (might) work, all of them include the time and space consuming (re-)installing of El Capitan. Since El Capitan should also have a internet recovery mode option it's unfortunate that one does not seem to be able to boot into that, if you have multiple (and older and "factory set") OS installed. (Option D)2., change the default start up disk to El Capitan and hit [(Opt +) Cmd + R] at the chime, should be working, but doesn't...)

So my Question is actually not solved completely, so I'll leave it open...
Thanks to every one who can give further advise! :D