Mix and match subs?

No, it does not work that way. One of the many things people realize when they get a DBA is how much extreme low end information there is even on recordings with zero low bass that can be heard as such. Similar to the way extreme ultrasonic high end cannot be heard as such, yet does create a greater sense of realism on all recordings, extending all the way down into the low bass region.

These simplistic views of sound and what we can hear are really, uh, simplistic.

Generally speaking, it doesn't matter (unless one of the subs is really terrible).  I recommend you take a look at Rythmik's line of subs for music.  SVS makes good subs also, you won't go wrong either way.  More is better when it comes to subs, unless you're in a really small room.

I’d go with the bigger second sub.  As mentioned above, matching exactly isn’t as much of an issue with subs, but that added bass down to 20Hz or so could add some real magic in an expanded soundstage and better imaging along with better depth.  Again, I’d try a PB1000 Pro because it’s literally free to demo at home and the value is very high.  Best of luck in whatever you decide, but I think you’re really gonna enjoy having two subs no matter what you choose. 

It is better that the subs match for proper integration with music due to tonal and sound pressure variations due to driver and cabinet size which effect the sound greatly.

Strangely I've done both. My second sub was the same as my first and they are placed at the front. After reading about the benefits of four, I purchased a pair for my rears that might be considered inferior. To be honest, the improvement is terrific even though there is a mismatch.