Isn't the answer to the first one xlim?
[quote]Xlim is expressed by Eminence as the lowest of four potential failure condition measurements: spider crashing on top plate;vVoice coil bottoming on back plate;vVoice coil coming out of gap above core; or the physical limitation of cone. A transducer exceeding the Xlim is certain to fail from one of these conditions. High pass filters, limiters, and enclosure modeling software programs are valuable tools in protecting your woofers from mechanical failure.[/quote]
The second bit, if you have the cabs, try it and see, get a long cable and wander about, to se if there are bad cancellations. The people madly criticising mixing cabs and drivers and such are either people in pro audio that are very fussy on the minutiae of good sound, or people who have read their statements and taken it to mean the world will implode of you go against them, because they are experts. In most cases, you won't cause many real problems mixing cabs. Some stuff is going to upset totally modelled stuff, but is fine in practice, for example, if you do the highly reccomended pair of 2x10s stacked on their sides, the bottom one is radiating in half space cause of the floor, but the top one isn't, so they are mismatched.