Do you guys think that in order for someone to lavabend they need to have both earth kingdom and fire nation heritage or be an Avatar in order to do it?
Probably not, but it is a distinct possibility.
As far as we know Ghazan (the lava bender of the red-lotus) is pure earth nation, meaning probably not. That being said it's possible that he does have some fire nation ancestry without knowing it.
It's worth noting that avatars seem capable of lava bending. This doesn't necessarily mean however that they use both fire & earth bending to accomplish this however it could simply be due to the avatars detailed understanding & high skill with bending.
Lava is also simply another state of earth. We see that water benders are capable of fluidly transitioning between Ice and Water with ease so it stands to reason that earth benders should be able to do the same (though it would be harder due to the greater temperature difference simply requiring more energy.).
In my personal opinion (though this is in no way cannon at all) lava-bending probably requires cross-discipline knowledge, much in the same way that lighting-redirection is based on a water-benders understanding of ebb and flow lava bending probably requires a fire-benders understanding of passion and energy but no actual "fire bender genes" the avatars all obviously have this, Bolin gets it from training with his brother and zaheer gets it from studying with the red lotus.