Some of the higher quality bulbs have rectifier circuitry that corrects reverse voltage.Yes they will not work if they are installed with the incorrect polarity.
Well yes, you need a LED flasher relay to run LED taillights and turn signals.
However, that relay won't do anything about the fender flare side markers (assuming there is LED bulbs in those). Those are the ones with the floating ground. While there is a way around it, you have to be good with electrical stuff, and I'm not, so my solution was simply to remove the LED bulbs from the side markers. Just not worth the hassle IMHO.
Wouldn't the fender bulbs have to be stock? The side markers can be LED and the fender bulbs halogen, but not the other way around in my experience. The only way I could see a halogen sidemarker working with an LED fender bulb would be if the fender LED is effectively a short circuit when the opposite voltage is applied.
The LEDs in the fender simply don't let enough current pass through the circuit to illuminate the halogens. In addition, the fender LEDs may be illuminated at the incorrect brightness due to the voltage passing through the halogen sidemarker bulb.