Will a large variable power supply burn out a heating element?
The Styrofoam Cutter needs a supply of 6 V, 2.5A

What you need to do is:


Make sure you supply 6 V to the  Styrofoam Cutter as 6 V is what it needs
Make sure that the supply is capable of delivering at least 2.5 A as that's what the Cutter will draw.


As long as you supply the right voltage then the load (the Cutter) will draw the current that it needs. The supply then needs to be able to deliver that current.

You mention that your supply can deliver up to 25 A so that is more than sufficient to supply 2.5 A to the Cutter.

If there is a current limit option on the supply then you can optionally set that to a little above what the Cutter needs so in your case I would set it to 3 A to 4 A. Than if the Cutter develops a fault and short-circuits at least the current is limited. With your 25 A supply that is important as 25 A is a lot of current.