Yeah at 30 tons it isn't going to fit. Not that the weights listed help. The Clan Gauss Rifle is 12 tons. Even if 1 ton of ammo were included, where does the extra .5 tons come from?

Looking at the weight for the Mad Dog, removing the extra DHS, keeping 3 tons of ammo, and removing the Gauss Rifles and 1 tons of ammo leaves 27 tons. That leaves weight for two Rail Guns weighing 13.5 tons each.
The Gauss Rifle's damage is also half what it is in BT. Doubling the damage would mean that the Rail Gun should do 40 points of damage.

Scary!
About the reloading. Does the Rail Gun take longer to reload? If we double the Gauss Rifles reloading time that's a 10 second turn. The Rail Gun's would be 15 seconds. So the Rail Gun would fire every other turn. Slower rate of fire but those hit will sure feel it!
So far I'd go with
Tonnage: 13.5
Damage: 40 (Can fire every other turn.)
Ammo: 5 shots per ton
Heat: 0?
Crits: I'd go with at least 7.
I can see why the SLDF would have been experimenting with the Rail Gun. I'd be lighter than the Gauss Rifle and hit a lot harder. The bug is that it doesn't fire as fast. Still, as bugs go, I think it's a minor one. I can see the Rail Gun being put into production.