Might be.

Here is a link that might be useful: spider mite images

Your understanding of the size and habits of spider mites is correct, Tiffany; you wouldn't be able to easily see them swarming over the potting soil. Plus, they don't live in the soil.

I'm guessing that you have one of the velvet mite species, more apt to be found in organicy potting mixes than, say, clover mites which are very visible on hard surfaces and on plants. Both are much larger than spider mites.

There are many, many species of mites...most are not interested in plants. Unless you have reason to believe that your tomatoes are being adversely affected, there is no reason to use insecticidal soap, which can alter the consistancy of your potting medium, not in a good way.

By the way, spider mites are a common pest of tomatoes and you should keep your eyes peeled for them. A simple magnifying glass can help you examine the underside of the leaves, where they hang out.

Thank you. I'll keep an eye on my plants and see what happens.