Thus far, avoiding NDA breaking, three confirmed companions have been mentioned for the IA.
SCOPRIO - The Droid revealed at E3 2011
Kaliyo Djannis - Rakkatti Bounty Hunter shown as the IA starting companion.
And Rania Temple - Mentioned by Kaliyo during one of the BioWare companion videos, unconfirmed who she is.
I believe each class will have 4-5 companions at launch, although I haven't bothered to check for the quote.
On the main point of the thread. Praying for an HK-51 Assassin Droid companion; the HK-51 droids were reprogrammed by HK-47 during his invasion of the HK Factory in the KotoR II Restored Content Mod, which, as the name implies, was a mod that restored content that was previously cut by Obsidian due to time restraints. Since the Light Side ending for Kotor II, which is apparently canon for SWTOR, requires the HK-51 droids to be activated to aid HK-47 against G0-T0 on Malachor V, we can pretty much guess that the ending to the HK factory was canon as well, meaning a large chunk of HK-51 units were produced.
Minor lore point to explain what the HK-51's were aside. Would honestly suit them pretty well as a companion. They were designed to be disguised as protocol droids to aid with their blending in. They were incredibly efficient killers, being based off of HK-47 and to be honest; who wouldn't love a companion with that sort of personality.
Praying for an IA HK-51 companion. Shame it'll be unlikely to happen :/
HK-47 himself may actually be in the game, without breaking any NDA there has been an official video about the discovery of a revan tomb at the end there is a very familiar droid that says "Interjection: Silence meatbags"