Errr... No.
Buy halo reacj armor in the startup menu in the armory for credits in online games
For now, no. Although, in the near future a new kind of armor might be introduced, much like Recon armor was in Halo 3.
its something called skills
It is highly suspected that there will be new armor permutations in Halo: CEA because of some screen shots that have been released from official sources.
To get security armor in Halo 3... all you have to do is get all of the original 49 achievements (you don't need the new mythic or legendary achievements).
yes, you get JFO armor, and you get mark IV armor, and also a gold visor but that is the max rank.
it alredy came out
Technically no, the only way to wear armor in halo 3 that you can wear in reach is some of the basic armor that was already available but the new pieces of armor such as the security shoulders, sapper chest with robot arm, an gungir helmet are only available in Reach.
You have to get certain achievements that unlock certain armor permutations.
You have to collect skulls and achieve in-game awards to help get points (which add up to new armor).
Not that anyone is aware of, 343 Industries may release DLC containing armor for Sangheli's (Elites).