I recently did a session with a 22/13/18 Slingerland kit and the engineer said that 18 was his favorite floor tom that he has EVER recorded. I would have to agree. It sounds amazing. Coated Amb on the bottom and Coated Emp on top.
It actually fit very well with the 13. It wasnt a giant leap between the two tone wise. For rock at least I think the 18 will do you just fine!
Just my 2 cents!
Adam