Thinking about getting a SSD for my laptop

So I've been pondering the idea of buying a SSD drive for my Lenovo G570 laptop, but I have a few questions.
Firstly, I would like to keep my current 750GB HDD, I've read online that I can put it in the DVD drive's place (and I don't need that thing anyway), but will I need some sort of support caddy to do that? Something specific to my laptop model?
That's pretty much the one thing bugging me. And also, are all SSDs the same size? I mean can I buy one and then choose to put it in either my desktop or laptop?

Thanks