does the metal slide is necessary ?
 

Ive been told but I dont know. those TM plastic GBB better to use the metal slide for safety, cuz if you use the original plastic slide, it may crack or something, so is this true ?

TM plastic slides are actually quite robust. Although not designed to run on it, most will function just fine on propane/green gas, without issue. If it ever does break, new ones are like $15, or just replace it with a metal one then.