You should forget about the power ratings when thinking about what can and what cannot be hooked up to each other.
Most of the time, you want to hook up one pair of speakers to a stereo amplifier. However, there are occasions when one may wish to hook up more speakers. In such cases, one should focus on the impedance that will be presented to the amplifier, as well as the impact of the interaction of the speakers with each other when hooking things up in series.
If the speakers are all the same, then it is simple: Hooking a pair up in parallel halves the impedance, and hooking them up in series adds the impedance. If you hook up three in parallel, you have 1/3 of the impedance of one of them; 4 in parallel gives you 1/4 of the impedance of one, etc. Again, this is how one can think of it if the speakers are all identical.
If they are different, however, then things get more complicated, as the nominal impedance of a speaker does not tell you the impedance of the speaker at any particular frequency. Speakers vary in impedance by frequency, and the "nominal" impedance is supposed to be what you can pretend it is overall when matching it up with an amplifier. But this is going to matter if one is hooking up a bunch of different speakers in a complicated manner with some being in series with some, and in parallel with others. And this is a good reason to not do it.
For some more reading on this subject, see:
Electrical characteristics of dynamic loudspeakers - Wikipedia, the free encyclopedia
Wiring Loudspeakers in Parallel | Audioholics
If you have more questions, ask.