I'd like to start a guild for EC, but I have several questions.

First and foremost, will you be able to have different factions within 1 guild? I.E. Can a Space Marine be in a guild with an Ork?

If not, will each character have the option to join a seperate guild. I.E. Will I be able to have a Space Marine in Guild A, and an ork in Guild b.

If different factions CAN be in a guild, what sort of interation, if any, will be allowed?

I think it will be more likely that separate characters will be able to join separate guilds (your second statement)

That's what I'm expecting too. And that'll be a pain as I'll have to start 4 different guilds. :/

Or join a non faction specific guild and use team speak or any other program to coordinate and play same faction and switch around however

Well, that's kinda my question. Will faction-ambiguous guilds be a thing within the mechanics of the game?

A Space Marine/Ork guild would be astoundingly lore breaking.

Every race in Warhammer hates every other race and commits unspeakable atrocities on each other as a matter of routine, so probably not.

Maybe when the dark eldar are put into the game they'll be allowed to join eldar guilds (and vice versa).

Well, a Space Marine bleeding itself out faster is, generally speaking, "astoundingly lore breaking."
It's a video game, and certain liberties are taken, because of that. For instance, Space Marines have to eat and drink, but it's probably not going to be in the game. Though it would be cool for buffs.

I think generally speaking that if there's Dark Eldar and Eldar on the same team that it's because they've defected. Ie, craftworlder abandons the path system and becomes a debaucorous fiend or a DE abandons the dark city in search of something more than hedonism. They don't drink tea and chat about how much better they are than the mon keigh. Even if they're willing to fight the same foe.

As astoundingly lore breaking as an Ork/Space Marine guild? It's not even in the same ballpark, and as other people pointed out(And I was too derpy and tired to remember in the relevant thread.) the blood clotting doesn't seal a massive chainsword through the torso, that you can actually make it go faster is easier to dismiss then fighting shoulder to shoulder with a Greenskin regulary, rather then a 'come the apocalypse' sort of thing.

Also, the suit recycles your waste.