Solaris VII rules were supposed to be used during "arena" games, not normal multi mech engagements. Mechwarrior series was based on the CBT (Classical BattleTech) board game which allows customizing as well as building new mechs from scratch. While it may lead to creation of some... well... monsters, it's fun and adds to gameplay. Besides, if you simply want to customize a bit, nothing holds you back - you don't have to build a multi PPC behemoth and you can stick to "canon flavors". So what is the point of taking this possibility away (if I recall correctly devs were explaining this as "it's weird to see lasers shooting from LRM racks")?
Ah, and as for small laser firing as often as gauss - keep in mind that in CBT board game 1 turn = 10 seconds. Don't try to imagine 1 turn of small laser fire as a single shot (as a matter of fact, multiple war games use "1 turn of shooting != 1 shot" principle) - it simply means that during one turn small laser will do that much damage and produce some heat, regardless of how many times it really did shoot. The same applies to ammo - 1 ammo round means that a weapon can fire during 1 turn - be it one gauss shell or designers-know-how-many machine gun rounds.
Now that I think about it, I wonder if the fast firing weapons in MW aren't more powerful than in a board game - they fire more often and probably deal the "1 turn damage" during a single shot...