The Beta 12 is about 99dB wihle your mid is 87dB... its not going to be nearly loud enough. Besisdes, that 12 goes up to 4k. the tweeter probably goes down that low.If you really want a cheap midrange, check out parts express.
and get something more efficient.
If your tweeter really doesn't go down to 4k, maybe try the Beta 12 LT, it can reach 7 or 8k easy.