Not sure where QuilterInVa got that info about Medifast & exercising. According to the official Medifast site & their support materials, it is recommended that you only exercise for 45 min/day while following the 5&1 plan. The reason for this is that the low calories/carbs really do not support intense workouts...your body wouldn't have enough fuel! When first starting Medifast for those who are accustomed to regular exercise, it is OK to continue but for those who are pretty much couch potatoes, it is suggested to wait a couple of weeks while your body adjusts.
Keep in mind that Medifast was originally designed for very obese people who probably did not or could not exercise much meaning that you will lose weight without exercising. However, Medifast does recommend that clients exercise as soon as they are able to do so.
Re: mixing WS & MF. Lots of people do. Remember that the WS shakes are the only thing that is really interchangeable with MF products (i.e., similar nutritional stats). MF & WS are two entirely different programs with very different instructions. Be careful when combining the two as you may end up spending a lot of $$ and not get very good results. Using something like FitDay to log everything in can help with making sure you are meeting the nutritional/program criteria for MF when using WS products.
My suggestion is to follow Medifast using just their products for one month. If you do their plan as written you will see your biggest weight loss during this time. Your taste buds will change and your body will adjust to the new style of eating. Then you can experiment with substituting some WS products.