I normally rule that for armor to apply to a flamethrower or something of the like it needs to be thermally sealed in some way. For the rock-people it's really your call, are they fleshy on the inside or completely made of stone?
In game it is strongly implied that they lack internal organs as we understand them, so I'd think they were completely made of rock.
Here's what I'm thinking of going with for them (using the SciFi Companion wherever it contradicts the Deluxe Explorer's Edition):
Environmental Resistance (Heat/Fire) (+1)
ArmorX2 (+2)
Size +1 (+1)
No Internal Organs (+1)
Slow (-1)
Obese (-1)
Outsider (-1)
The Rock people move at half speed in the FTL game, but have 150% normal hit points and are immune to (environmental) fire damage. Culturally they are known to be extremely xenophobic.
This write up gives them +4 Armor, +4 Armor vs Heat/Fire damage, a total of +2 to Toughness (+1 from Size and +1 from Obese), a Movement of 3 (+1d4 Sprint), and -2 to Charisma to most other races. They are also immune to extra damage from called shots.
Their effective defense against fire damage is 8+(1/2 Vitality) if we don't count their armor against fire, 12+(1/2 Vitality) if we do.
Working on the other races; current hang ups are how to portray the Zoltan ability to power ship systems and how to handle the Mantis ability to do double damage in combat. I THINK I've got a handle on the rest of them, and will post them here in time for review.