This command installs flash for Firefox.
I'm not sure about Chromium-Browser, but I think Chrome comes with Flash built-in.
sudo apt-get install flashplugin-installer
I don't bother with the latest and greatest Skype, but use the version in the Ubuntu repositories
Firefox and Thunderbird should install without unsafe packages. Please try with the following commands
sudo apt-get install skype
and report back whatever output you get in the terminal (copy and paste and put within code tags (the # icon at the top of the editing window)).
sudo apt-get install firefox
sudo apt-get install thunderbird