Where can I find guidance or standards for environmental storage conditions for new and older legacy media types?

While not directly answering your question, I would recommend that any data stored on legacy digital media be copied to a more robust storage solution as soon as possible in order to prevent (likely) further degradation.

If the media itself is to be preserved then I would defer to the storage standards provided by the traditional preservation community of which I have unfortunately minimal knowledge.

The Northeast Document Conservation Center (NEDCC) has some great free resources about object conservation. They recently came out with a primer on digital preservation as well. From their site (http://www.nedcc.org/), click on 'free resources' on the far right.

I'm not a preservation/conservation specialist, but (generally speaking) the colder and dryer and darker your storage area, the better off you'll be. For specifics on temperature and relative humidity, I would look particularly at the NEDCC's guidance for audio recordings and/or video as a baseline for magnetic media.