I have wargamed in 1/72 for a long time, and most of the time mixing 1/76 is not a problem. I would only suggest that you don't mix scales of similar tanks.
For example a friend used 1/76 kangaroo carriers but then 1/72 shermans and it looked a little odd.
Otherwise mixing the scales is usually the only way to fill the gaps. Certainly the general plastic figures look fine next to 1/76.
Though if you have any concerns you can always check out
http://www.plasticsoldierreview.com/Index.aspx which has a scale guide with each review. And you can see from it stuff like the valient plastics as lovely as they are, are larger then 1/72 really. So while i have found you can mix them with 1/72 vehicles at a push they look plainly silly next to 1/76.