[RPG] What are the rules for forced movement into and out of water and flying

From the DMG/Compendium, the important part for your question:

Forced Movement and Terrain

Difficult Terrain: Forced movement isn’t hindered by difficult terrain.

Blocking Terrain: Forced movement can’t move a creature through blocking terrain (page 61). Every square along the path must be a space the creature could normally occupy.

Challenging Terrain: Forced movement can make some powers more effective or hinder them, depending on the specific challenging terrain. The DM can require the target of forced movement to make a check as if it were moving voluntarily across the terrain, with the same consequence for failure.

Hindering Terrain: Forced movement can force targets into hindering terrain. Targets forced into hindering terrain receive a saving throw immediately before entering the unsafe square they are forced into. Success leaves the target prone at the edge of the square before entering the unsafe square. If the power that forced the target to move allows the creature that used the power to follow the target into the square that the target would have left, the creature can’t enter the square where the target has fallen prone. If forced movement pushes a Large or larger creature over an edge, the creature doesn’t fall until its entire space is over the edge. On the creature’s next turn, it must either move to a space it can occupy or use a move action to squeeze into the smaller space at the edge of the precipice.

A DM can allow a power that pushes a target more than 1 square to carry the target completely over hindering terrain.

Published in Dungeon Master's Guide.

And from the DnD FAQ ( http://wizards.custhelp.com/cgi-bin/wizards.cfg/php/enduser/std_adp.php?p_faqid=1396 ):

Are zones that deal damage (like the Wizard power Stinking Cloud) considered ‘hindering terrain’? Can I make a save to fall prone and avoid being forced into one?

No, zones are not considered hindering terrain. Hindering terrain refers to more permanent features like pits, cliffs or pools of lava.

The resultant answer: For zones, base DnD rules do not give a saving throw to avoid being put into the fire. For pools of acid, you are given a saving throw to catch yourself before it and fall prone.

Yes.

There are no limits* to how high a pixie may fly, provided that they end their turn only one square above the ground. However, if they do end their turn above their altitude limit they fall with the consequences laid out for falling (1d10 damage/10 foot of drop).

*The theoretical maximum height a pixie may reach on their turn (with no extra movement trickery) is 8*3 + 1 or 25 squares high (run 8, trade standard for another run 8, action point for another run 8, +1 for starting height), that's 125 ft at the end of which they would fall. Falling says to subtract the creature's fly speed so that leaves us at 95 feet of fall, meaning 9d10 falling damage.