Can you use summon monster X if you are fliying 1000 ft above the ground? or do you need to summon the creature in solid ground?
Look under the "Magic" section at the descriptions of the different schools. Conjurations that call or teleport things need to be on an open, unoccupied surface- even if the creature/object can fly.
Elemental Plane of Air, the summon-free plane.
Indeed. It just swaps Conjuration with Transmutation and Necromancy to change Air into *things* and brings their spirit into it, if applicable.
Wait a minute, that means that summoning or teleporting underwater is imposible?
The rule is probably to prevent people from summoning bison/orcas/t-rexes onto things to destroy them with the falling object rules. If you aren't whale-bombing the enemy or teleporting the ground underneath the BBEG into the air over him for an instant burial, it's probably not going to be overwhelmingly important.