First post, so bear with me if I goof this up

I'm considering buying a 990/7500 pair with a set of Onix Rocket speakers. The mains are 8ohm. The center is 4ohm. The dipole surrounds are 6ohm. Will this create any kind of strain on the amp? I know that sometimes people are concerned about leaving channels without load or balancing load such as with vertical biamping. So, if I have this mix of 4, 6, and 8 ohm, will it cause reliability problems?

If the answer to that question is no, would I expect to have to adjust channel gains on the prepro to compensate for the differing loads? I know that the lower loads receive significantly more wattage (even though it is more difficult for the amp to drive). Am I to expect the 4ohm center to be putting out a much hotter signal than the 8ohm mains? Most of the other speakers that I looked at such as Paradigms seemed to have consistent impedance all the way around on a 5 or 7 channel setup...so these Onix speakers throw me off a bit.

I guess a final quick question might be (if any are familiar with the onix speakers)...would the 7500 be a good amp to drive Rocket 760s, 200 center, and 300 surrounds? Just curious.

Thanks to anyone that may respond. Take care.
-Andrew