I have some questions about Dragon Age 2;a moment of your time?

I've just recently completed Dragon Age 2.

It felt so awfully short! I'm left wanting more. I really enjoyed it.

So, in my endeavour in doing more, I ask these questions:

Is Mark of The Assasin on Xbox 360?

How do I recruit Isabela and Sebastian?(I was really frustrated when I learned I had missed these two)

Is it possible to get different clothes for your companions, or was that just a mod I saw? I watch DanaDuchy's channel on YouTube often, and she had most of her party wearing cool dark robes and things lol(I love Fenris' high-collared suit).

I apologize for the barrage of questions, but I would be extremly grateful if they were answered! :)I have some questions about Dragon Age 2;a moment of your time?
MotA is downloadable content, available on ANY console. You'll need a net connection for your Xbox, and money/points/credits (whatever Xbox uses) to pay for the download of the game, which isn't that much. When it downloads, install it, and enjoy :)
There is also Legacy, another DLC you can get, which came out just before MotA.

Isabela is in the Hanged Man during Act I, keep going back after every few quests, and she should be there. Complete the quest she asks you to, and she will be recruited.

Sebastian needs to be downloaded. He is part of the Exiled Prince DLC, which came free with the special edition of the game, but you can download him and his quests just the same as you do with MotA and Legacy.

Putting companions in different clothes are to do with mods, which are only available on the computer and not console. You can improve companion's armour, however, by finding improvements throughout the game. This gives them better specs, but the outfits do not change.
When you romance a character, their clothing also changes slightly. Fenris gets the Hawke/Amell crest on his belt and a red token around his wrists, Isabela's corset changes, Merril's armour changes, and Anders' robes become black (though I think this happens for Anders whether you romance him or not, as long as you complete his quest - not sure though).

As well, on MotA (haven't played it yet due to broken net connection), I'm pretty sure your party members wear fancy clothing (for reasons I'm sure are explained in-game) and so this is where you might have seen the high-collar suit. If so, you'll be able to enjoy it again when playing MotA as I think they're default clothing for all party members.

Hope this helps!