Can I install the Android M Developer Preview on HTC One? So I just wanted to try out the new Android M developer preview on my HTC One M8, but the only build available is for Nexus devices.

If I flashed it on my HTC with MultiRom, and it bricked, could I easily just remove it with MultiRom? Or would it mess up my main stock 5.0 ROM also?

In that case, should I make a Nandroid backup with TWRP?
The M Preview images are for nexuses only so it wont work on any other device, although if you really wanted to try out Android M there is a possibility that a developer somewhere takes interest in developing one for your phone as soon as source code is released but you will have to wait though. You should be able to remove the ROM from MultiROM but its always advisable to take a backup when changing ROMs.