Whatever are side and rear surrounds must be:
Matched
Mono pole (based on experience and advice from respected acoustics experts like Nyall Mellor and others, any other radiation pattern degrades performance because modern HD audio depends on directionality in the surrounds...the more diffuse the surround radiation pattern the less accurate is effects "steering"....also highly dependent on proper placement/height)
Site mid/treble drivers per Dolby's current protocol, at proper angle and height only a few inches above listener's seated heads
I don't personally know any benefit to surround bass cutoff lower than about 80-90 Hz. Considering budget constraints, ask Jim for a dome 2-way appropriate to match your front mains. He probably already has at least one.
My system performance dramatically improved trading four matched quasi-bipoles @ 7' height to four matched mono poles sited just a few inches above seated ear height.