I have a 1998 4.6 with 5 speed manual transmission.
I was bleeding the clutch and the slave cylinder was wiggling a bit both when I was opening and closing the bleeder valve, and when the clutch was being depressed.
Is it normal to have the slave cylinder wiggle a little?

 

Are you sure it was the slave and not the bleeder pipe? The way the pipe is pressed into the slave it is possible for it to move. There is a spring on the slave that pushes the release bearing against the pressure plate fingers. There is no way that you will overcome that spring with an 8mm wrench on the bleeder valve.
Pull the rubber plug and shine a light in the opening and have a look at the mounting bolts.

Regards

Jean Marc Chartier

EDIT: Spelling

 

Thanks... will do.