How similar are Crested and Gargoyle geckos in terms of personality and care?
Care yes, personality no, Gargoyle are not as laid back and calm they have a lot more personality.

Are Crested geckos happier housed with other Crested geckos or by themselves?
Never house 2 males together but females are fine, now they can be house together but I recommend to wait until they are adults to do so for various reasons. If you house young cresties together one will be bullied and eat less and therefore grown slower, leading to more bulling from the now bigger cagemate. You also want to be careful not to house a male and a female together at too young of an age, if you chose a pair, so you do not end up with an eggbound female, even if you chose to house a male and a female together when they are the right size the female should be given a break (keep in mind that once a male is introduced they can lay egg up to a year or so even after the male has been removed and do so once a month).

^ What about Gargoyle geckos?
Best to be house alone at all time except for breeding.

Pacman frogs: Should they be housed separately? How often/what should you feed them?
Can they be held at all? (I know that they're skin is very fragile)
Definitely better alone, they are so voracious they will not hesitate to eat their cagemate. They can be held just make sure that your hands are very clean and free of soap, fragrance etc.

How much space do each of these animals need?
18 x 18 x 24 for single Crestie or Gargoyle as adults, but something smaller with young individuals so they can find their food easily (ideally a small critter keeper)

Pacmans do well in a 10 gallons tank or 28 quarts tub.

Does eating mice really negatively affect a tarantulas molt??
No idea I do not do inverts