I found a fix somewhere, which made me understand I didn't need to have S-Off to have a Custom OS, flashed, rather, there was this small trick, booting the phone from the boot.img file with adb instead of rebooting from a custom recovery. This was an HTC Sensation, not a Samsung, Only trying to help:
* Get/Extract the boot.img file in your custom rom's zip and save it somewhere on your desktop.
* Once you're done flashing the custom rom in recovery, DO NOT reboot from recovery, turn it off by removing its battery and fixing it back, then boot your phone into downloader mode and run these commands.
adb flash boot boot.img
Hope I was able to help.