If you have good grounds you don't need isolators. If you have alt whine, you still don't need them. They're a bandaid for bad grounds.
Most car speakers are rated for 4 ohms, most decks as well. If you feed them a 2 ohm load you may run into trouble. Rms doesn't matter if you're sending them less. The rms is just what they are supposed to be capable of carrying for extended periods. Unless your deck puts out exactly what your speakers are rated for and you're constantly at full volume your speakers will always be seeing less watts and are constantly under-driven.
You can overdrive a speaker, within reason, if its a clean signal. Few decks regardless of rating will send a clean signal at more than 10-15 watts. After that they're sending distortion, and distortion kills speakers in short order. a good amp will send more clean watts and the same speaker will live a long and healthy life even though it sees more power.
Imo you want your speakers to be rated for at least and preferably slighly more than what your system can produce but you won't hurt anything if they can handle 100w and you feed them 20.
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