Adding parallel WP theme to site

I am considering adding a parallel WP page to an existing WP site. This would be to add a guest blog with a different look to our existing site. I was going to import a new theme into a separate directory and link to it. The original site is an earlier version of WordPress because of a desire to stick with a certain gadget – of course the newer theme would be a newer version Are there any unforeseen problems that I may not be considering. I don’t want to wreck what is already in place.