This, are the standard rules that most people like to follow, if you do not know them, do not battle:


Sleep clause:
No more than 2 pokemons can be under the sleep status in the oponent team.

Species clause:
You can not use two pokemons of the same specie in one team.

Evasion clause:
Do not use moves to increase you evasion, this are moves like double team.

OHKO clause:
You can not use OHKO moves, some are sheer cold and fissure.

Tiers:
Do not use pokemons of a higher tier than the specified tier for the battle.
Here is the complete list of the pokemons and their tiers.

No hacking
Do not hack for battles, self-explanatory.

No D/C
Do not disconnect in a battle, it's not fair.

All of these rules apply in any battle, unless you specify not to, and you can always add some custom rules.