Allopex Reaver

So, I've been thinking about using one of the new Idoneth Deepkin models, the akhelian allopex, as the arena champion for my reaver squad. With a heat lance taking the place of the harpoon launcher on the back, and the crew kitbashed to be more like jetbike pilots. Chains grafted onto the shark itself, as well as a trophy rack or two if there's room, to really make it look like a creature broken and altered for the rider's own needs.
There's an issue, however: The allopex is in all likelyhood much larger than a reaver jetbike, and I'm afraid that could cause some balance issues when measuring charge distance, etc. Is there any way to get around this?

Regardless of the potential issues when it comes to the tabletop, I think I will still do the kitbash just for fun.

Yep.

Now someone please do correct me if I'm wrong on this, but I do believe this is correct.

DIstances are measured from the BASE, not the model.
THe only exceptions are vehicles which specifically have a rule on them which says "measure to the hull".