I do apologise people but I must put my ore, or virtual palm over your mouths in a minute. First off, I am sad the elites are not apparently playable as they were a species i enjoyed (due heavily from the assassinations in REACH) but I can live without them in multiplayer etc.
As for the Elites and their weapons, I feel too many fans are simply going by what they believe and not researching the Halo CANON. The director is rather close and although I have skipped a few posts he is also the only one, besides Ms Misty I might add, who has a sane grasp on this universe.
Firstly, the San'shyum (forgive my spelling I do not write the xeno names often) we know of are the descendents of outcasts exiled from their own planet. They used the Dreadnaught (the ship in halo 2 and 3 that powered the portal) as a warship when they began a war with the Sangheili. This war began due to a major issue between both sides.
Both races saw the Forerunner as Gods, but where as the Elites idolised but refused to "abuse" the devices of the gods, the Prophets saw them as gifts to be used. Of course this meant it was almost as though someone was having a rock concert in your local church to the elites and although my knowledge on the fullscale war is vague I am informed well enough that the elites won the majority of the ground battles and the prophets (in no small part due to their Forerunner dreadnaught) the space and air battles. Eventually the elites were forced to "descecrate" the relics themselves in order to match the technology of the Prophets.
Ofcourse this meant the elites were performing the acts their enemy had done to start the war. Slowly both sides came to realise the war was, in lack of a better word, pointless and eventually made a truce which grew into an alliance. Eventually forming the foundations of the Covenent itself. The Prophets would become the political leaders in civil and religious affairs while the Elites managed all military ones.
The Hunters (species name I can no remember) had a similar situation. The elites found themeating the gods devices, such heracy meant the elites tried to wipe them out. Eventually it was realised that this worm like beings had a fascinating military advantage and were eventually permitted to join the covenent.
Grunts were (and I'm adding this for the hell of it) permitted to serve in the military due to their devotion and ferocity during the Ungoy rebellions.
If you have any questions please do not hesitate to ask. My sources are primarily Halo Waypoint, Halo wiki and Contact harvest the novel.