Homebrewers - Whats in the fermenter?

[jonumberone]

Hey guys.
Just getting started, and my first batch is bubbling away.
I already got a ton of help from Albert and Tim, but I don't want to be burdensome.
I have a few questions and I was wondering if anybody can help.

I plan on doing 2 more 5 gallon extract batches tomorrow.
How much water should I start with.
My first batch I started with 5 but finished with a bit under 4 gallons of wort.
I thought about adding water after the fact but didn't want to screw up, so I left it alone.

another question.
The other day I unpacked my 5 gallon carboy from the box.
It bears a striking resemblance to an empty 5 gallon water jug.
I have access to plenty of these jugs, about 25 a week to choose from.
Could I use them as a disposable secondary?

Thanks in advance fellas!