Building additional template for WP page

I am interested in having an additional template for . I hired someone on fiverr that strung me a long for a week and then said he'd prefer to redo my whole site so I cancelled the gig.

Here is the situation:
I have as my main site.
I have installed WP Symposium (it is live, but not viewable to users yet). Our sociala network is called The Sunroom. (It's a former Ning site, but I am not importing)

I would like to have an additional template to choose from for WPS pages--even if I must set them by hand. I know the pages that have already been created I will have to redo by hand, but not sure if I can have future pages created within Symposium default to that template.

Basically I want the pages of WPSymposium to have a different sidebar widget area and a different logo.png header at the top of the page. If it is possible, I would like the background color to be different, too. Menu navigation bars, etc can all remain the same.

My WPS skills are beginner/moderate.

Any suggestions? Should I post this as a job to hire someone or is it simple enough I should give it a shot? I read a blog post here on a plugin that can make it possible to use 2 different THEMES on one site.

FYI: My site is NOT a multisite. I looked into this in the past and it seemed more thank needed, but now I am having second thoughts if I am going about this wrong. I figured I would ask for help before It am backtracking too far.

Thank you for any tips/advice.