The umbrella is not necessarily too big for the flashes you mentioned if used indoors. However, you might want to try this flash bracket with the setup:

http://www.idcphotovideo.com/store/t...at-traditional

The problem is that the light coming from a single flash may get diffused a little too much. I've always tried to have a multiple-flash bracket on hand for those times I needed some extra punch in my mainlight. With such a large surface area to light, a multiple flash bracket seems ideal.

Regardless, make sure you have a heavy duty lightstand and even a sandbag or two on hand when using a 7ft umbrella.