Heat is also the big other concern, as 'Mechs will rapidly overheat under normal circumstances, causing them to force-shutdown. Energy weapons generate enormous heat, as well as missiles, while ballistics generate less heat (normally). Handling your weight in the 'Mechbay and your heat on the battlefield is the real core of gameplay. You try to get both of these as streamlined as possible, and combat equipment comes naturally from the excess space/heat you learn to shave down. Energy is generally not a concern, as it's assumed beam weapons have their own generators (probably nuclear) with enough charge to last through any given battle. Ammunition for ballistics and missiles, however, is a very real concern.
In-universe explanation for energy weapons is that they pull directly from the Mech's fusion reactor. Another protip: when deciding on weapon loadouts, the general idea is that you want a big damage weapon like a PPC, high-caliber autocannon or large laser, followed up by "spread" weapons like missiles, machine guns, LB-X autocannons or a slew of lasers.
The reason is that once an armor section has taken enough damage to drop it to zero, further hits will damage the internal structure and give you a chance at critical hits for
each point of damage. Unlike a lot of FASA's other products (like the
Renegade Legion line or even the
Battlespace complementary system to Battletech), damage is recorded as a straight numerical value rather than an armor template. Ten points is ten points, regardless of whether that's one PPC hit, ten LRM hits, five SRM hits, ten machine gun hits, etc. When you hit with a missile salvo, every five points of damage is rolled on a semi-random table for hit location. So the more hits you get, the more chances to hit a location where the armor has been breached.
So ideally you want to punch a few holes in an enemy's armor with the big guns, then focus on spraying them with small multiple-hit weapons to increase the likelihood of critical damage. A critical hit can completely knockout a weapon or other system, torso critical hits can damage the servo or engine (both of which are major hindrances), and lucky cockpit critical can injure or kill the pilot outright.
You can either try to balance that in a single design (like the Griffin, Thunderbolt and Orion) or --and this is where the real mastery of the game begins -- in using multiple specialized mechs well together. A Warhammer + Trebuchet + Spider, for instance. The Warhammer uses its twin PPCs to punch holes in an enemy, which the Trebuchet saturates with missiles, while the agile Spider keeps harassing the enemy by manuevering into its rear arc. (Rear armor is far thinner than frontal armor, so that even light mechs can cripple a haevy or assault if they can get in the rear arc and stay there.)