What steps did you do to set-up the page level test that uses content from multiple templates? When I try to reproduce that I get the template validation restriction. Also, what version of SC?

I'm going to take a stab at answering this generally, since I don't have the particulars of your specific issue.

All tests in Sitecore (Page tests, content tests, component tests, the tests that run as part of personalization…) are based on a single test definition template. The variations that I'm aware of are those related to editor presentation and workflow options.

If you've figured out how to hack the page tests to use different templates, and you're getting test results out of them, I can't envision a scenario where you'd run into trouble. Especially, because tests are designed to run for a short period of time; we test, we learn something and repeat. We're not (at least, not yet) tracking the history of tests & test results.

The last comment relates to having to select from different templates. Templates are typically tied to data sources. And, we don't typically test different content against one another - an article against an event… If you're Sitecore architecture requires that you make template variations to affect presentation, there's probably a better way -- Component based design.