So I use a hot plate on my Peavey 5150 EVH and the dude that works at guitar center was telling me that it will ruin my amp.
Is this true?
I'm using the 16om hot plate
No it wont. This is a common misconception.
What actually happens is you will go through valves faster than if you weren't using it.
This is not because of the hotplate though it is because of the settings you are running your amp at. If you ran it at the same settings as you do with the hotplate normally you would go through valves at the same rate.
If anything it actually protects your amp somewhat too as if you have a failed speaker cable or a short the hotplate could very well save your transformers.