Pet stores are not typically the group you want to emulate in your home setup. They are often more concerned with their bottom line than the health of the animals. Many pet stores that sell reptiles also hand out misinformation. After countless times of hearing, "that snake won't outgrow the tank you put it in", "yeah, you can feed a cornsnake crickets", and "that green iguana won't get that big and can be fed iceberg lettuce and cat food", I tend to discount 90% of the 'information' that they dispense. I'm fortunate in that my local pet store that specializes in reptiles is also in possesion of a knowledgable staff that genuinely cares about the animals' well being. You won't see species mixed in their tanks and you'll find accurate information from the majority of the staff. Those of the staff that don't know the answer to your question will find someone who does. That's not to say there are no stores in my area who do mix animals, there are, they just don't get my business. As to my thoughts on those stores who mix animals and give out inaccurate information, I either try to educate them to care for the animals properly or I make sure to let every herper I know that they need to be avoided at all costs if they refuse to correct the problems. We as consumers have the power to make changes if only we make a stand. All too often we just roll over and let the poor practices continue and knock it up to 'status quo'.
Fact is that different types of creatures carry natural loads of bacteria which vary between species and habitat, not to mention that when mixing animals of different sizes you run the risk of the larger animal intimidating the smaller one and causing it to decline in health. You may house different animals together and not have any problems, or you can put two together and spread disease and parasites between the two, or severly stressing one individual from forced contact with a larger animal - potentially losing one or both of them. I won't say I've never mixed species, I have - a ball python & a Colombian rainbow, both still in my collection. I was told since they had basically the same requirements and were the same size, it was fine. Luckily I never had any problems, even after years being housed together, but never-the-less I took an unnecessary risk out of ignorance. These days I've learned more about proper husbandry & quarantine practices, and strive to provide the best environment I can for my animals. After all, the hobby is supposed to be about the animals...and it's our duty as their keepers to provide the best care we are able. That includes being sure to take responsibility for the housing and proper care of every animal in our care...including the ability to provide proper housing for each. It's fine to cut some corners - racks instead of more spacious individual cages for juvenile or small animals, multicage units to save space with larger animals. But some things stay the same, quality of life is of the utmost importance - proper housing, correct husbandry, enough food & fresh water, security. For me, it's all about our responsibility and commitment to the animals we keep - whether that animal is a common ratsnake you found in the back yard or the latest $20,000 morph on the market.
Sorry for the long disertation but this isn't directed in any negative way towards you. I simply feel very strongly about our commitments to the animals we keep and feel that some corners just shouldn't be cut for our convenience or whim.
Off the soapbox now. 
Raven
