Welcome to Envato, or more specifically, Themeforest (as you’ve mentioned it).
1.) If you’ve a website with WordPress, than yes, it’s best to stay with WordPress themes.
Unless you’re comfortable developing those static templates into a WordPress theme.
From the sounds of it, I do not believe that’s an option right now so stick with themes for WordPress.
2.) You can buy templates/themes and use them for whatever your mind can imagine.
We’ve bought themes for clients and developed them in ways far from the demonstrations use.
You could buy a theme for Fitness and use it for Gaming or vice versa.
Depending on your capabilities, in most scenarios, a theme can be considered as a building foundation. It’s the framework you’ll use to piece together your website. Maybe you enjoy a certain theme for it’s unique layout, but you’re not a big fan of it’s color scheme or themed niche. You can change that to whatever you can imagine and it truly being your only limitation; your imagination.
So if you want to buy a Brides website and use it for a Sports club site, by all means, go for it!
However if you’re looking for simplicity and convenience, buying one themed for your purpose may be the easiest solution in your moving forward without a lot of struggle. Again, it’s entirely up to your imagination.
3.) If the theme/template demonstration only previews 6 pages, than that’s all the pages they’ve pre-built. Those pages will likely come as demo content when you buy that theme. They’re pre-made laid out page layouts / ideas that you can use in moving forward after your purchase.
HOWEVER, this does not mean you’re limited to those 6 pages. That’s all the author decided to demonstrate. If you want to create additional pages, by all means, go for it. If you’re using WordPress, then use the page editor to create new pages and those will appear as you construct them. If using an HTML template, simply save the page file (usually .html) as a new page name (ie., started as about.html and now created a second copy, services.html). It will vary based on the type of website you’re building (ie., WordPress, HTML, Drupal, etc).
Hope this helps.