If I switch from regular Rampant to Rampant Fixed after playing a scenario for a few hours, will that mess anything up? Will I still get the full experience of Fixed?

In terms of functionality - no problems
but if the evolution is 50%+ - the defense may not be ready for the mix of biter factions. In about 10 - 20 minutes, the existing nests will mutate into new ones

I've only played my present scenario a few hours and already my defenses are being overwhelmed. The original Rampant wasn't making bugs that have 100% resistance to lasers, but your Rampant is. Since my defenses are entirely laser-based...sigh.

Yes. Had to do it. Unfortunately, some weapons from mods can have laser damage in the tens of thousands. But laser biters are very vulnerable to physical damage, and they can also be killed with flamethrowers.

Specifically, these biters are dealt with by the standard flamethrower+laser combination. Or just regular ammo-turrets