This wouldn't allow you to increase your number of targets with hotter fire, though. (I still think 0 should be valid, by your reasoning that "up to" means "less than or equal to," and 0 is a number in modern counting systems.)
Well, Behind the Ferns and Spirit of the Panda are both ongoing spells ("enchantments" in MtG terms, even if one would be an aura and the other not) with continuous effects, right?
So if you have BtF first, the Huntress gets Stealth, then you play SotP on it, and it gets +2/+2, but the BtF is continuously checking attack power, so the Huntress immediately loses stealth, right? (Even if you are applying both of these continuous effects after or "on top of" any continuous or triggered effects created by the unit card itself.)
Steam Tank is a different matter. I would argue that Steam Tank loses Stealth once it attacks a building (as soon as it gets the ATK bonus, the BtF Stealth falls off), but it doesn't matter because Stealth only limits target selection, not what happens after target selection, and it already selected a legal (at the time) target.
Though I really appreciated @Coiser pointing out earlier that the Stalking Tiger's wording is really funky and doesn't seem to work with the Steam Tank ruling. I would say that Stalking Tiger's ability should be rewritten or ruled to really say:
"Stalking Tiger can attack units as if it had Stealth (all detector rules apply when doing so)."
This fixes the timing, because it doesn't require you to choose your target before getting stealth; it instead says that you are always treated as having an ability that is similar to stealth, but with a more limited range of attack targets (backline units, but not backline buildings/heroes).