Is it possible to root directly custom rom? I have bought Xiaomi Mi 5c (bootloader unlocked) and tried to root it. I think I've used every possible software available on internet for rooting (like TWRP+SuperSU, iRoot, KingRoot), even tried to modify a system and boot image from custom rom by placing there SuperSu app and su binary. Also I have used some script which patches sepolicy and ramdisk with tools available in SuperSU app and do other necessary things to root the device, but after that my device does not even boot (stuck on fastboot).

I was wondering if I can use the same script (with some small modificatoins) to root directly a custom rom and then flash the whole custom rom with available root inside.

Intuition tells me that this should be possible, but till now I did not found any custom rom with root inside (except dev chineese roms which are already pre rooted, but that's a different story).

I want to root directly a custom rom and then flash it as I hope this could be the last chance to make it to work. So again, is it possible?
MI 5C is a fairly new phone and with Xiaomi's in-house SoC (harder to develop for than established ones like Snapdragon), therefore custom ROMs are expected to come out later and in fewer quantities.

Stock MIUI also employs a check on /system partition so as to prevent booting when it's modified, no matter how it's executed (modified before or after flashing).

There are a few tips on XDA about rooting, but I've examined them and none of them seems to be surefire. At this moment, the Developer version of MIUI is your only dependable choice. Since you're already unlocked, flashing it should prove no trouble.