A: Figuring out which Keepers to treat as equivalent is something you just have to use common sense and discussion for. Before combining any two decks, take a look at the Keeper set, and see if any things seem similar enough to be considered equivalent for the purposes of Goals.

You should treat combining Fluxx decks the same way you would with any House Rules, making sure everyone playing knows the quirks of this particular set. If you think you might combine two given decks more than once, or leave them combined, you might consider writing down some of the equivalencies you decide on, for future reference.

A couple of easy examples would be deciding that the following are equivalent:
The Sun in Fluxx 5.0 or Remixx,
Sunshine in Nature/EcoFluxx
The Sun in Astronomy Fluxx

The Moon in Fluxx 5.0 or Remixx
A Small Moon in Star Fluxx
The Moon in Monster Fluxx

A more complex example would be this list of potential equivalencies to Keepers in Dog Fluxx, which a fan, Huey Makt put together:
Adventure Time! Fluxx: Jake the Dog (any Dog)
Camping Fluxx Fluxx: A Sharp Stick (A Really Nice Stick)
Cartoon Network Fluxx: Courage the Cowardly Dog (any Dog), Jake the Dog (any Dog)
Cthulhu Fluxx: The Cat (any Cat, Black Cat)
Doctor Who Fluxx: K-9 (any Dog, if all players agree)
Family Fluxx: The Ball (A Ball), The Cat (any Cat), The Dog (any Dog), The Mouse (The Mouse), The Stick (A Really Nice Stick)
EcoFluxx/Nature Fluxx: Mice (The Mouse, if all players agree)
Hundred Acre Wood Fluxx: Some Sticks (A Really Nice Stick, if all players agree)
Oz Fluxx: Toto (any Dog)
Star Trek: Porthos Expansion: Porthos (any Dog, Beagle), Spot (any Cat, Orange Cat)
Wonderland Fluxx: Cheshire Cat (any Cat), The Dormous (The Mouse)