ProtoMechs don't really work with IS tech. Or at least they'd be poorly armored and armed, not really worth it perhaps. This assuming there'd be appropriate stat changes like heavier armor and heat sinks, etc.
That said, i like this idea, and if you go digging, you'll find that people have come up with variety of ideas already.
I know, the rules thread's linked in my sig. :P I was just using IS Protos as an already-existing example to spur people's imagination.
As for their effectiveness, I dunno, I think these little things could be reasonably successful, even with their drawbacks. There's actually a little stack of scraps of register tape with hand-written theorycrafting on them right next to my monitor, and about 1/3 of them are IS Protos of various flavors, mostly using the 60 SFE, 120 SFE, and 45 TFE engines. Most of them seem like they're comparable (albeit inferior) to Clan stuff of the same tonnage. I just never got around to putting them into the computer is all.
Heavy Lasers wouldn't work well with IS tech due to heat sink size issues. The Blazer is kinda of a proto-Heavy Laser as it is.
Mreh, I've kind of taken a shine to them myself ;)
ATMs would be wasted for the Spheroids as they make extensive use of special munitions so MMLs are natural fit for them. YMMV if MMLs would be wasted for Clans though, they can use special munitions as well after all.
Maybe the Clanners would wind up being the ones to use lots of special munitions, then?
Clan VSPLs would be kinda hilarious, assuming they get proportional range and damage boost. Though unless their weight is also lowered, they don't really offer that much over standard pulse lasers and targeting computer combo (though this same applies to Spheroids pulse lasers).
I dunno, I guess the weight *would* be lowered a bit. Maybe 1.5/3/8 tons? At the cost of adding a bit of extra heat over the IS version, of course.