So, i believe the General rule is ALL UNITS on ROUND BASES can make a close combat attack using their Primary damage value
-- Infantry (even if the infantry is hover, wheeled, Vtol, etc...) as long as the target is touching front arc.
-- Mechs (which are also on round bases) can use their Primary Damage value for Close Combat. Even if there is a "CC Fist" on the secondary the primary Damage value could be used and CC SE (brawling could be used). This is why the BR Legionaire was able to CHARGE for "6" Clicks of Damage. 4+1+1
-- ARES CLASS also can make close combat attacks.
-- "In addition to the combat values shared by all other ’Mechs, colossal ’Mechs also have tertiary damage values and heat effects"
---------------those slightly different ------------------------
Quad mechs -- notice, they are not on a Round base
--If a ’Mech with the (QUAD MECH) speed mode doesn’t have the (CLOSE COMBAT FIST) range type associated with either of its damage values, its primary damage value becomes 1 for that close combat attack.
(SO, even though Quads CAN CC ATTACK THEY MAX AT 1 DAMAGE)
Vehicles--- must have a special equipment to allow a paticular type of close combat attack ie Flamers or Repair coolant
--FLAMERS (optional) This unit may be given close combat orders, regardless of its type.
--REPAIR (optional) This unit may be given close combat orders, regardless of its type. Make a close combat attack targeting a friendly figure
--(purple) COOLANT FLUSH (optional) This unit may be given close combat orders, regardless of its type. Give this unit a close combat order targeting a friendly figure not in base contact with an opposing figure. The attack does not score damage, and it ignores all non–heat effect close combat modifiers. This unit may move up to its speed value before making the attack. If the attack succeeds, the target loses 4 heat if this unit didn’t move before making the attack, or 2 heat if it did move.
hope that helps :D