Machinegun turrets loaded with incendiary ammo will destroy themselves somehow. I suppose they burn biters which come too close and flames burn down turrets thus making it near damn impossible to use this type of ammunition in defence. Is it supposed to be this way?

Incendiary and other (e.g. High Explosive) RA's magazines have AOE damage applied.
Wall off your turrets from biters. Walls are 100% fireproof.

Less important factor, but still about walls: Vanilla turrets have no resistances at all (vanilla factorio problem that could be addressed by RA by the way), therefore biters destroy those turrets very quickly in any case.