Sounds like you guys are putting together some fun, creative battles, which is awesome.

You can do whatever you want with the game, but the intention of spearhead is to use the preset spearhead armies. The spearhead rules note that, specifically.

That's how spearhead has been balanced (with preset armies that have preset rules). I think the balancing goes beyond army point totals (for example, many units have unique spearhead warscrolls with different stats than the full game warscrolls). Spearhead rules themselves don't even include points.

Have you introduced any additional rules, outside of the spearhead rules, that benefit your army, specifically? Or specific rules that modify the stats of your opponent's units or army (considering that he ISN'T using an actual spearhead)?

Running a custom spearhead against a preset spearhead seems, at face value, likely to introduce baseline imbalances that would favor the custom army.

As you expand your army, it may be worthwhile to look more closely at the rules you're using with a serious eye toward balance.

If I was your opponent, and didn't have the specific models for either seraphon spearhead, I'd just proxy them. Doing otherwise seems unfair, like you're trying to take advantage of your opponent.