I'm really only interested in Space traits that stack. There aren't a whole lot of them.

Leadership: As far as I can tell, this trait stacks. With no diminishing returns with two BOFFs with this trait. Unfortunately, I only have two officers I can use to test this. Does anyone know what the maximum stack is on this trait?

Efficient: I've heard this stacks, but I only have one officer that has this trait. How much does it stack? Honestly though, this trait doesn't appear to be useful if you already have Warp Core Efficiency maxed out. With a rare Saurian with Efficient, it only increased my Aux power by 1 (from 39/25 to 40/25).

Covert Operative: Does this stack or not? I've heard mixed results on it.

Not sure about Leadership, I began playing this game when the trait did nothing, so I never really kept any human crew. Easiest way to tell would be to equip one at a time and look at your passive hull regeneration to see if it goes up or not.

Efficient does stack without diminishing returns and without cap (to the number of officers your ship can have), but its effects are a bit low to the point it may look like they aren't doing anything (only adding fractions of a power point). They're still worth it if you don't have anything else, every bit of power makes your ship a little better. It doesn't matter where your Warp Core Efficiency skill is, the Boff trait will add the same amount of power at the same power level regardless.

I don't have a Covert Operative yet, so I can't comment on that one. If it adds direct points to a skill, it probably stacks without cap like Efficient.

Yes, it works. I only had one human officer assigned, and I was getting 20 to Subsystem Repair and a total of ~122% hull repair rate. When I added another human officer for testing, it went up to 40 Subsystem Repair and a total of ~142% hull repair rate.

I was pointing more to the fact that 7.5 Warp Core Efficiency doesn't seem to do a lot. My power levels are 125/90 weapons, 74/55 shields, 52/30 engines, 39/25 aux. With one officers, I have 40/25 aux . I'm just not convinced having 5 Saurians is going to give me that much. From what it's looking like, I may get 2 or 3 boost to engines, and 4 or 5 more aux.

It's not a bonus to a skill. It's +2.5% critical hit and -10% to cloak ability cooldowns. I forgot to check to see if this BOFF was a unique officer. But, there's a male and female variant. And an uncommon and rare variant. It appears that the Very rare variant shares the same ability as the Science and Engineer variants and does not have the Covert Operative trait.

Well, my fleet mate said he could stack the Covert Operative trait.

I tested it myself, and can confirm that it does. I had 7.2% critical hit before, and now I have 14.7%. With a Zero-Point Energy Conduit, my fleet mate said he was able to get it to 16.4%.