6x7 only gives you 10 shots per roll. For candid stuff you're hit rate is going to be lower than for formal stuff so you might find yourself burning a lot of film, or worse missing shots while you change film. A 645 gives 16 shots, and if you get something that can handle 220 then 32 shots per roll will save you a lot of fiddling. Obviously quality isn't as good as it potentialy could be with 67, but it's WAY better than 35mm.
I'm not sure through the lens focusing is essential - with a good flash you can stop down nicely, and pre-focus at a couple of meters. I've had great fun doing candid stuff at parties like this with a TLR (far more candid than an eye-level SLR). It limits your composition a bit, bit you catch the momment.