Most mecha are divided into different pieces; the Chest, a number of Arms, a number of Legs and a Head, each with a set number of slots. You can add additional parts onto the mecha by using a number of slots, listed in the item's description.
For example, a mech's arms might have 6 slots, and a certain energy blade would cost 3 slots. This would fit, and the mecha could be equipped with the weapon. However, if the arm is already hosting a built-in cannon, costing 5 slots, the energy blade could not be built-in, but could be held by hands, which cost 1 slot. It's assumed that common sense applies here. If you have something that physically would not fit, and cannot rationalize it at all to your GM, he should be able to say no. Also for things like shoulder mount weapons, you can use points from the frame and arms, split how you like.
Other mecha do not use such humanoid parts. These can be anything from tanks, to VTOL, to a Capital Class Warships. No matter what they are, they can be purchased with a frame for the right style of ship you want. They will have more slots (usually), and some parts will not be able to be purchased.
Each part also has HP, which is listed on the mecha sheet. I'll be adding to the combat page for rules regarding this.
The breakdown of different Races' styles is here.
Human - A variety of Japanese mecha (think Gundam) and westernized designs (think Battletech).
Kyurgan - Large amount of ornate craftsmanship, with heavy armor, and powered by Kyurg crystal energy cells. Think Protoss.
Ra'Zalga - Metallic and usually crafted after bipedal animals or angelic/demonic figures of some kind, Ra'Zalga mecha closely resemble human's mecha, with more emphasis on craftsmanship than utility. Think Advent.
Sithrugami - Using unique biomechanical weapons and armor inspired by the blight, these mecha look freakish and are commonly look like blighted humanoids. Think... Uhh... Zerg-Infested Terran Technology. Or Infested in Warframe, kinda.
Vraekin - Vraekin mecha use energy from the beings that possess to power themselves, as well as a creative use of the Ether. Their model resembles steampunk mecha settings. Think Chaos or WarmaHordes.
Yythrilm - Using more primal-style mecha, these are the most primitive looking, but due to their high ether-run energy, they're actually quite effective. Think a mix between Golems in fantasy and WamaHordes.
Mecha Armor: Mecha Armor (noted by an M after the number) can be treated normally by Penetrating Damage (noted by the Penetrating tag. A weapon will list Mecha Armor Reduction, and normal Armor Reduction separately, and most Infantry weapons have none at all), or, each point can be penetrated by 50 points of regular damage. So that pistol you have isn't gonna take down the Frigate. Sorry.
//I'M FUCKING SORRY DID I JUST BORROW RULES FROM CTHULHUTECH? WHAT IN THE HELL WAS I THINKING?
//No, no, no, no, nonono, and no. I'll fix this.