Installin El Cap - Partitions and Such

My rMBP13 running Yosemite is my main machine. I want to try out Yosemite to make sure that all of my work applications are compatible with El Cap, so I've been reading about installing it as a separate partition so that I can safely play with it.

Here's my question: if I do that and everything is okay, do I then just uninstall/delete the Yosemite partition? Or is there a way to combine them? I've never done this before so I want to make sure I am doing things right. Or is there a better way altogether?

 

Might be a dumb question, but having never done any partitions I don't know:

If I install El Cap on the second partition while Yosemite is on the other, will El Cap have access to all the installed programs and all the files on the Yosemite side?

And even though I can keep two partitions, I would actually like to combine them if the El Cap release works with all my stuff. Is it just a case then of deleting the Yosemite partition? Or upgrading Yosemite side once the public release comes out and then joining the partitions together?