Well, it depends. BattleTech weapons are insanely powerful. They have to be, to get through the similarly insanely protective BattleTech armor. For example, a modern 120mm hypervelocity tank cannon is a Light Rifle (Cannon) in BT terms...meaning it does no damage to 'Mech armor. This would suggest that Leviathans weapons would be just as worthless against BattleTech units...and BT weapons would start in the range of 3DBlack and go up from there (way up). Worse yet, given how vulnerable leviathans are to lightning (electroid "flashes over" catastrophically when over-energized), any hit from a PPC would likely blast great holes in a lev and drop its carcass like a stone.
Really, the tech bases are just so vastly different that neither system would work well for crossovers. This isn't a bad thing, it just reflects two very different realities (each with different divergences in physics) seperated by over a thousand years.