In areas such as the Battle Frontier, the game puts restrictions on the Pokémon you can bring in. The Pokémon banned from the Battle Frontier in Emerald are:
Mewtwo
Mew
Lugia
Ho-Oh
Celebi
Kyogre
Groudon
Rayquaza
Jirachi
Deoxys
Egg
In the competitive battling scene, Pokémon are categorized into different tiers. The most used tier is the Smogon Tiers. The tiers are Uber, OU (overused), BL (borderline), UU (underused) and NU (neverused). Only NU Pokémon can be used in NU battles. NU and UU Pokémon can be used in UU battles, all the way up to Ubers where all Pokémon can be used.
Note that tier lists change from generation to generation as Pokémon gain new movesets or counters.
So to answer your question, Latios can be used in the Battle Frontier.
You can find Rayquaza in the Sky Pillar, in the upper portion of route 131. You'll have to go there when following the main quest when you'll have to "wake up" it after the start of the fight between Kyogre and Groudon. You won't be able to catch it then because it will fly away, although you can actually go back there once the fight between the other two legendaries stops and try to catch it. It will always be lv. 70.
Best Answer
You can get it from the Devon President in Rustboro City after having delivered his letter to Steven in Dewford Town.