if you decided to grow a bit of fruit or veg along side them, like tomatoes or grapes etc, might as well make most of the space unused if it doesn't effect your main crop
no, it won't affect your mary jane. Plants can only cross-pollinate between like genus.. for instance citrus and citrus... lemons and limes... cuccumbers and squash... only other cannabis plants can pollinate your cannabis plant.
If the other plants in there flower like weed then up might have issue with the new plants flowering when you dont want them too. It wont hurt the weed though.