Currently, I have a site that has many subjects on it. It's more of a test site and results in the SERPs have been mixed. Each subject has an average of three pages (linked together). Lately, I thought about writing about one particular subject and will have dozens of pages with lots of keywords to it. I was wondering, if you guys were me...should I stick this on my test site that has lots of other subjects on it? OR, should I build a new site and have this one subject with many pages on it? Just wondering if the new site would rank better in the SERPs or if it didn't matter, as long as you have lots of pages w/ rich keywords and linked together properly. Also, really don't want to put out extra $$$ for a new site...any thoughts which way is better?
Take the one with the older domain and start working your SEO on it. That should make your decision easier.
Firstly, it's always better to have sites that are at least vaguely focussed on a particular topic. That said, if you are short on funds, there is absolutely no reason why you couldn't create a new section in your 'test' site and get it ranking. You would need to create a sub-section for your new subject - like a mini homepage - and build lots of optimised links into it. Try to limit the amount of links that go out to other sections of your site - keep it as focussed and close-knit as possible - this will ensure the section retains it's authority without spreading it too thin across the whole site.
If the site is already loaded with many topics then it is advisable to take new site. As suggested by Jennsellsfast its better if you take older domain.
I generally prefer to keep different subjects on different domains, rather than all of them on one. That being the case, there is absolutely no reason why you cannot put all your subjects on one site ( most online newspapers do just that, and rank for each section quite comfortably)
Generally speaking, it's better to bundle sites. Google is emphasizing more and more the total trust of a given domain, so when you bunch all your content together, you bunch all your links together, which creates an overall more powerful domain. However, reasons to split the sites include being able to get a keyword rich domain, branding, and bad user experience.
Here is another question then. Currently, every article I write, is linked from the index page. So if I write on this particular subject (that I have dozens of pages on), should I make it into a mini-site, and not have the sub-pages linked to the index page? Shouldn't there be a flow where the mini-site isn't just hanging off the main site, not connected to the front (index) page?
Google is able to categoriza the main theme of your site, so its better to start a new fresh site if you are thinking seriosly on this new content.