For the purposes of game effects? Example: the Arborec's Bioplasmosis reads as follows:

"At the beginning of the Strategy Phase, you may move 1 Ground Force from a planet you control to a friendly or uncontrolled planet in an adjacent system.

(Discard any Domain Counters on the planet)."

Now I understand the obvious exception above, but does that only apply to unrevealed Domain Counters? Once a Hostile Locals counter has been revealed does the planet then count as "controlled"?

Also, are you able to do the same but to an uncontrolled planet in a system you currently occupy? Meaning, if there are two planets in a system, and you successfully occupied one of the two planets, can you use Bioplasmosis' effect on the other planet?

Thanks and sorry if this has been addressed before, I couldn't find anything about it.

 I think the planet is uncontrolled since no player has units or their control marker on the planet, and thus no player is in possession of the planet's card.

Yeah, the hostile locals would be plant food.

"Damn nature! You scary!"

Excellent, that's what I figured.

Also, how about Winnarian Custodians? I would imagine then it does count as being controlled since they're not technically Domain Counters. And Mecatol Rex usually has exceptions to other rules surrounding it (no pun intended…that was a stretch).

In general, a planet is considered uncontrolled if no player has the corresponding planet card in front of them. Mecatol Rex is, as almost always, exempt from this generalization.

Yeah, I think the Custodians effectively "control" Mecatol Rex at the beginning of the game if you're playing with them in the game. I look at it as a sort of "scenario rule" that overrides the usual rules. In this case, the rule about what counts as a controlled planet. While the Custodians option section of the rule book doesn't explicitly state this, it seems reasonable.

Would it slow down play to go for a cautious yet effective "Scan n Grab" early game? Essentially scan every near planet and just grow on the bad planets (to ignore the penalties/combat) whilst settling the nice ones like normal?

It would definitley be a slow but effective way to spread.