How can I run multiple requests dynamically using the same parameter each time?

I'm not quite sure I understand your problems, but I think you are saying you already have created 25 Test Request steps and you want to insert a property expansion in each one without doing it manually?

I would probably back up the project xml, open it up in a text editor file and do a search and replace (standard disclaimers apply!)

Regarding your second issue, if it was the same value you were after in each request, I would do a Property Transfer to a Properties step, then reference that Property in all of the requests.

Hope this helps.