Yeah, you're right. My guess is they are 11 tons (w/ ammo) as the Uziel without JJs has ~23 tons of weapons, and the Harasser (given a fusion engine instead of ICE) has ~ 12 tons of weapons available.
Lt. Gauss Fafnir would still be overweight by at least 5.5 tons though, and it would change my suggested design as such.
Chassis weight - Assault - 100 tons
Internal Structure - Endo Steel - 5 tons
Engine - 300 Vlar - 19 tons
Gyro + Cockpit - 6 tons
Armor - Standard - 19.5 tons
Pod Space - 50.5 tons
1 Lt. Gauss Rifle - RA - 11 tons (1 ton of ammo included)
1 ERLBL - LA - 5 tons
1 ERMBL - RT - 2 tons
1 ERMBL - LT - 2 tons
1 Lt. Gauss Rifle - RT - 11 tons (1 ton of ammo included)
1 Lt. Gauss Rifle - LT - 11 tons (1 ton of ammo included)
6 Free Tons
LAMS - 1.5 tons
BAP - 1 ton