I'll answer these in order:-
Battle spot - Random Matchup, Free battle - You can use any Pokemon here except including legends, if above lv 50 - all participatns get rounded back to lv 50, if below lv 50; they're entered at that level (of whatever they were). Same goes for Doubles, Triples and Rotation.
Battle spot - Random Matchup, Rating Battle - Everyone starts at 1500(pts) unless you previously players and lost/won.
This information was copied and pasted from HERE Copied and Pasted because it gave me information on the subject.
Below is a list of the basic rules that are used in most of the Rated Battles: •Any Pokémon with a level greater than 50 will have its level automatically set to Level 50. Pokémon who's level is 50 or below will not have their level changed.
•Players cannot use two or more Pokémon of the same species / same Pokédex number.
•Players can only use any Pokémon that can be obtained through regular gameplay or distributed through an event. Pokémon that have been imported using hacked methods are not allowed.
•Players can give their Pokémon items to hold for use in the competition but no two participating Pokémon can hold the same item.
•Players can only use Items that can be obtained through regular gameplay or distributed through an event. Items that have been imported using hacked methods are not allowed.
•None of the following Pokémon may be used: Mewtwo, Mew, Lugia, Ho-oh, Celebi, Kyogre, Groudon, Rayquaza, Jirachi, Deoxys, Dialga, Palkia, Giratina, Phione, Manaphy, Darkrai, Shaymin, Arceus, Victini, Reshiram, Zekrom, Kyurem, Keldeo, Meloetta, Genesect, Xerneas, Yveltal, Zygarde
In Order to Participate in Online Competition (be warned) season 3 already started You would have to register at Pokemon global link >> http://www.pokemon-gl.com/.
Hope I Helped!