Sitefinity CMS URLs and IIS URL Rewriting - iis-7

Question

I have a site that is being served out of Sitefinity CMS, at http://www.example.com/, I also have sites on the same server that are being served out of the same root location that are accessed as follows: http://www.example.com/sub-site/. All of these sub-sites are in php and resolve to specific php files such as index.php. There are a lot of these sub-sites and we're looking to remove a number of them from the server. We usually send variables in the get request to these sub-sites as follows: http://www.example.com/sub-site/?address=12&ID=22
What we want to do is set up a rewrite rule that will redirect incoming requests to non-existent sub-sites to an error page written in PHP. This rewrite rule should keep the query string intact so that we can still use those variables once the error page is reached.
Here is the re-write rule that we have:
<rewrite>
<rules>
<rule name="RedirectFileNotFound" stopProcessing="true">
<match url=",*" />
<conditions>
<add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
</conditions>
<action type="Redirect" url="errorpage.php" appendQueryString="true" />
</rule>
</rules>
</rewrite>
So this rewrite rule works terrific, in fact, it works too well. When I go to the Sitefinity site's home page all is well, but when I click on any of the links on that page, I'm redirected to the error page. I've determined that the problem has to do with dynamic server content as the Sitefinity site links look like this http://www.example.com/order/ or http://www.example.com/info/. These directories don't exist and thus, the rewrite rule catches them each time.
Is there a way that I can redirect the subsites without having the sitefinity site respond the same way?
I found this S.O. post that is addressing a similar problem, but I was unable to find much assistance there.