Though I was wondering, can you combine the hardcore mod with the Final Mod Pack?
I am not aware of a mod called "hardcore".
If you meant "hardmode" mod (by hellrazor), then the answer is no, you cannot use both at once, they are not compatible.
In general, any two big mods currently in existence (as of March 2023) are incompatible.
You can only combine one big mod with some small mods.