How important is isolating food in a fridge?

The air movement in a fridge is required to keep the fridge at temperature. While it is true that more mass keeps a more even temp than less mass, you still need to keep enough room for the air to circulate. That circulating air will not contaminate the other food, although there are a few things you should do to maintain good food safety in your fridge.

• Place food in the proper spot The proper order to stock your fridge is (from bottom to top) uncooked poultry -> uncooked meat -> fully cooked meals -> raw, washed veggies. This order prevents any dripping bacteria reaching your food before you cook it. (Note that you might need to switch meals and veggies, depending on your storage needs. I use this order since I store all cooked products in sealed Tupperware but the recommended order has them switched)
• Make sure that your fridge is cold enough Temp range is 34 F to 41 F and you want it toward the lower end of that scale.
• Keep your fridge clean You should be cleaning your fridge, including the rubber gaskets around the door, at least once a month and more frequently if you begin to notice any kind of build up. Use a mild detergent (like dish soap) and warm water (you can also use bleach if there is staining or you are cleaning a spill), making sure to wipe out the sides and backs and the undersides of all shelves.

If you follow those three steps, you will have an almost zero chance of spreading bacteria around your fridge, with your biggest risk factors being your own hands. If you don't have enough shelves to keep each type of food separate, you can mix cooked food and veggies and the uncooked poultry with uncooked meats, but you should never store cooked food and uncooked food on the same shelf.

Unless they're aerosolized (e.g. by sneezing), or specifically designed to do so (like mold spores), food contaminants are very rarely airborne. All of your food comes with its share of bacteria, so the goal of the fridge is not to keep things sterile, but to greatly slow the growth of these organisms.

As mentioned above, the biggest thing you can do is keep your fridge clean, and put things away in such a way that things such as meat juices (where meat must be cooked to be safe) do not drip on things like salad greens, which may be consumed raw.

Plastic bags are great for some things (such as containing a drippy container of meat), but they will trap moisture around fruits and vegetables, leading to earlier wilting and spoilage. Better than plastic bags would be simple tubs (tupperware, glad, anything). Put different types of ingredients in their own tubs (wash regularly), keep fruits and vegetables in the open air in these tubs, but keep the wrapper on meat and cheese. This should contain any spill that could happen.

As for the air, unless you have something actively growing large mold which gets disturbed, there is little the air can carry that would contaminate your food. It may smell, and that smell may permeate some foods, but will not cause contaminations.