For quoting multiple posts in a single reply, use the button with the Quote Symbol and Plus sign.
Every reply you want to quote, hit that button and it will turn red. When you have all them red that you want, hit the Post Reply button.
As for doing multiple quotes in a sig, you just need to put whatever you want quoted into the brackets [ QUOTE] ..... [ /QUOTE] (without the spaces).
Originally Posted by The Pope
LA DI FRICKIN' DAH
is the equivalent of:
[ QUOTE]BLAH BLAH[ /QUOTE]
[ QUOTE="The Pope"]LA DI FRICKIN' DAH[ /QUOTE]
without the spaces. Also for the second one you can see I added a name. Something you can add in if you want.