Just verified in SSW... a 5/8/5 'mech can indeed carry a Thumper, 2 LRM-5s and two tons of ammo for each (more if you use less than max armor, or decide to go without the Medium Laser):
Fast Artillery 'Mech
Mass: 55 tons
Tech Base: Inner Sphere
Chassis Config: Biped
Rules Level: Experimental Tech
Era: All Eras (non-canon)
Tech Rating/Era Availability: E/D-F-E-A
Production Year: 0
Cost: 9,753,582 C-Bills
Battle Value: 1,095
Chassis: Unknown Endo-Steel
Power Plant: Unknown 275 Fusion XL Engine
Walking Speed: 54.0 km/h
Maximum Speed: 86.4 km/h
Jump Jets: Unknown
Jump Capacity: 150 meters
Armor: Unknown Standard Armor
Armament:
1 Medium Laser
2 LRM-5s
1 Thumper
Manufacturer: Unknown
Primary Factory: Unknown
Communications System: Unknown
Targeting and Tracking System: Unknown
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Endo-Steel 91 points 3.00
Internal Locations: 1 HD, 1 CT, 5 RT, 7 RA
Engine: XL Fusion Engine 275 8.00
Walking MP: 5
Running MP: 8
Jumping MP: 5 Standard
Jump Jet Locations: 1 CT, 2 LL, 2 RL 2.50
Heat Sinks: Double Heat Sink 10(20) 0.00
Gyro: Standard 3.00
Cockpit: Standard 3.00
Actuators: L: SH+UA+LA+H R: SH+UA+LA+H
Armor: Standard Armor AV - 184 11.50
Internal Armor
Structure Factor
Head 3 9
Center Torso 18 28
Center Torso (rear) 7
L/R Torso 13 19
L/R Torso (rear) 7
L/R Arm 9 18
L/R Leg 13 26
================================================================================
Equipment Location Heat Critical Mass
--------------------------------------------------------------------------------
2 LRM-5s RT 4 2 4.00
Thumper LT/LA 6 7/8 15.00
Medium Laser RA 3 1 1.00
@LRM-5 (Smoke) (48) RT - 2 2.00
@Thumper (40) LT - 2 2.00
Free Critical Slots: 0
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 6 Points: 11
5j 3 2 1 0 2 0 Structure: 3
Special Abilities: ARTT, SRCH, ES, SEAL, SOA, IF 1