Okay, so in your situation I'd want to try to capture as much of the ambient of the party as possible. I'd probably start off with your 50mm (80mm 'equivalent' and a nice portrait lens on crop) in manual mode: ISO 400, f/2.8 at 1/60sec. Chimp your screen for a rough idea of where your exposure is landing.
RE: flash I'd probably ceiling bounce or, like you suggest, use that strap-on softbox. I'd probably keep my speedlite in the hotshoe in second curtain sync mode - this is important because at 1/60sec you are going to capture subject motion blur, and you want their movement to be frozen by the flash at the end of the movement, not the beginning. I really like a little motion blur for parties, it makes people more animated.
The other issue you will have to deal with is mixing flash with ambient in terms of white balance. This might be particularly tricky at a Halloween party if there are 'spooky' lighting colours. But if you're indoors and lights are on, your best bet is probably going to be tungsten white balance with a full cut CTO to balance your flash. Since it's Halloween you might want to experiment with other colours on your flash like green or leaving it ungelled (which, with tungsten white balance will be a strong blue colour). Also, if you're able to get your flash off-camera you could light people's faces from below for a spooky effect ... but don't go nuts with that visual cliché.
That's a bit of a rushed explanation of what I'd do... but there's more than one way to skin a cat. Let me know if you've got any Q's.
EDIT: I forgot to mention that 1/60sec with an 80mm focal length is a recipe for camera shake. Subject motion blur with sharp surroundings is cool. Subject motion blur with blurry surroundings is awful. See how it goes, but you might have to ramp up the shutter and ISO if your hands aren't really steady.
On the other hand, with your Tamron at 17mm you could get some nice wide shots at 1/30sec without too much trouble.
I always leave it on second curtain anyways haha. I like capture the subject at the end of the fram as well. haha. Thanks a lot for the info and advice, I'll be sure to ask more questions if I need some more advice
i just can't believe the OP complain ISO 400 from a somewhat new T1i. especially you are printing 4x6.
i used to own that T1i, i use up to ISO 1600 all the time if i print 4x6, 3200 doesn't look that bad too.
i guess slight noise really bother some photographer.
Really! Go and get these shots à 800, 1600 or even 3200. You'll be blown by how much ambient light and atmosphere you'll get. I think that this is what you want from those party shots.
This will also use much less flash power and expand your battery's lifetime. Also, your owner's manual tells you that you shouldn't shot more thant 10 or 15 shots in a row at flash full strenght because it can overheat it or change the color of the flash glass cause of the heat... Another reason to put up the iso and open aperture as it requires less flash power and more safety shooting. Canon's flash don't stop by themselves because they're overheating, you have to be careful for that.
I agree with others, shooting in manual at 1/60 to 1/100, wide open to f/4, iso 800 to 3200, flash bounced off 90° or 60° and you will certainly have the results you want. I suggest TTL flash (metering Throught The Lens) and get the benefit of that 9-10ft roof. It will help you have a more diffused light on your subject as you know.
Also, turning your "to much grainy photo" in black and white gives an interesting result too. Kind of old looking photography. Give it a try!
Have a nice time
OH! LOL I DID COMPLAIN ABOUT PUSHING TO 400. I'm an idiot. Hahaha.
@MK5GTI - I meant pushing it up to 1600 haha. I have no idea what I was thinking when I was typing this out. It was like 2:30am xD
Well I've complained about the noise on my T1i just recently because I have felt the true low noise performance on a 5dmkII. I was just awestruck by the cleaninest of the noise well past 3200. That being said, a lot of my friends shoot with cameras that were made post 7d sensor so they have the benefit of shooting 1600 with no problem. I'm a pixel peeper and I know it's bad :x I agree, images are usable at 1600 on a T1i, but to my eyes it's just my nightmare. I don't like grain unless it's in Black and White (like what you said