As long as the speakers are at or above the rated ohm load for the speaker outputs of a device you are fine.

Using speakers that have a lower impedance than the rated output of a device =
1) It might work fine because the device can handle a little more load than rated.
2) You will damage the outputs of the device (or it will keep shutting off to try and protect itself).

In your situation, you have 8ohm speakers, so those will work fine on any of those systems.

The Aiwa CX-NAJ20 = It only has stereo outputs (clearly labeled at the bottom). The "Surround-Sound" outputs on the RCA jacks, are a gimmick. Original the system came with 2 little additional speakers that plugged into there - limited bandwidth with a delay/reverb. Those are NOT real speaker outputs you want to use.