If you didn't already purchase Mavericks, then it isn't available to you. Only Yosemite is available to you..
If the software to connect to the Flip video phone was PowerPC based or just better suited to 10.6, then you'll have issues with it on 10.10..
One thing to always remember is that as you upgrade OS', you have to upgrade all software that you have installed to make the transition smooth. Trying to get software that was meant to work on 10.6 (for example, which is an OS that was released in 2009) is just not going to be happy with something that is released 5 years later..
The wireless printer should be fine if its recent enough or has updated drivers (most reputable brands do a good job keeping up to date with new versions of OS X).
The iPhone 4S also matters what version of iOS its running. Be careful with that since iOS versions are happy with particular versions of iTunes. This is usually a problem if you can't upgrade your computer to a newer version of iTunes, but that doesn't seem to be a case for you here..
So to give you an idea of my environment. I have a late 2009 27" iMac that's running Yosemite, all the software on there that I use frequently has been upgraded to the newest version. I also have an iPhone 5S running iOS 8, an iPad 2 running iOS 8. I have a Canon wireless printer running the newest version of the drivers..
With all of these changes, everything is fine..
Now as far as hacker/security trouble..Yosemite is no more vulnerable than Mavericks, or Mountain Lion, or Lion, or Snow Leopard was. So, I don't push much stock in those bogus claims..