loop-de-luke

I'm building a custom quadbox cabinet which has 2 celestion g12t-75s in it already. I want to put in 2 greenbacks as well to beef out the tone and add some uniqueness.
I use a Marshall 1959 single lead plexi head (100 watts)... Will this be bad as the greenbacks are only 30 watts? I know you add the wattages together for the whole cab but I've never looked in to mixing different types of speakers before.

Also, how should I wire it up for best performance? I want to be able to use it alongside my 8 ohm 2x12" cab every now and then too...

Cheers