Hey Cosmic Charlie,
I don't know what your receiver is actually rated to tolerate as far as speaker loads goes. However, in general, there shouldn't be any problem with it handling different loads per speaker. Each channel is its own amp, and as long as each channel can handle a low impedance load, it should be fine.
Now, as to the low impedance load on the rears. The problem with low impedance loads is that they generally demand more current from the amp, especially at high levels. My take is that surrounds don't usually demand much from the amplifier -- most of us could probably get by on 10 or 20 W to the rears and be OK. Also, if your rears have a crossover applied (e.g., at 80 Hz or so), there will be much less current demanded from the amp (and bass requires more current than highs). I think you'll be OK.
If your receiver goes into protect mode, that would definitely be a sign that you are having a problem. I figure that didn't happen, or you would have said something. If your receiver is running very hot, it also may be a sign of a struggle to supply current, or it may just be the way the receiver runs (so it may not tell anything; you could probably do some temperature testing with and without the surrounds running to see if there's a difference).
I don't have a problem running 4 ohm loads to almost any receiver. I've done it all my life -- when I was a kid, I did it because I didn't know any better and I just wanted to hook up a bunch of speakers; as an adult, I do it because I know how amps work and current flows, and I know the limitations of my receivers or amplifiers. I have connected in parallel two sets of speakers: one is in my family rom and another is on my patio. That's definitely a four ohm (or less) load, and I have no problems from an old Kenwood receiver that runs that setup. As long as things are kept to a reasonable level, there's no reason for the amp to burn up.
Anyway, good luck. I'm gonna go play some Franklin's Tower now for the family -- thanks for reminding me!