Stats for such weapons are found in Tech Manual. For example, the Man-portable Plasma Rifle does 1.58 damage to anything it hits.
For finding out if flame-based weapons have any special properties, consult the rules for infantry construction: If a platoon mounts 2 or more support weapons per squad, it gains the benefit of the support weapon's range brackets and any special abilities. In the Plasma Rifle's case, you gain the option of dealing heat or damage to heat-tracking units. Just like 'mech Flamers, the amount of heat applied is equal to the amount of damage you would normally do.