Running a t5ho ballast short a few bulbs?

I have a couple 6 bulb t5ho fixtures I haven't been using because they would screw up my heat levels.

Can I run them on 2 out of the six bulbs? Will that hurt the ballast or create a fire risk?

 

They are hydrofarm lights. here's the manufacturers link
Hydrofarm - Hydrofarm FLT56UN T5 6-Tube Fixture (Bulbs not included)