If the overall Q on the other subs is different then the Sub 25 which I would think it will be, the lesser quality subs may muddy up the sound of the extra clean Sub 25....
Now the question is whether or not you will like the sound of them paired together.... I am not familiar with the BPS line...
People suggest sticking with the same subs to match SQ - but it is possible that it may not be bad with them working together... Ultimately - I am an experimenter so I tend to like to try things out - trial and error if a big fav of mine... I have tried out many different drivers in purchased products and ultimately in my many different builds.
Now, I do have a suggestion for you.... instead of dropping major amounts of money on that Sub25 - how about you build your own 18" LMS - I would suggest a pair.... For the price you will pay for 1 Sub25, you can have dual LMS's that will easily blow away the Sub25's in output and SQ.