For the most part, Fastpasses are available, but your return time may be as many as 5 or more hours later in the day. Mission Space and Test Track run out before noon. The rest of the rides will still ahve Fastpasses for the majority of the day, and sometimes into the night. You'll get your most attractions done earlier in the morning and late in the afternoon. Fantasyland ALWAYS seemed crowded, kinda nightmarish. To keep a level of serenity, make sure you "do" those great attractions you sometimes pass over even though they are great: Tom Sawyer, TTA, Swiss Family and others that gobble up crowds or don't attract as many.
If you are staying at a Disney Resort, I would def. take the mid afternoon break. Crowds are very bad in the afternoon. We usually skip out by 11 and return after dinner time to a different park, so yes, park hopping is good.
