Is it possible to root directly custom rom?
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.