Amp/Head unit wiring

I need some help with wiring and have a few questions on what I need to do to add an amp to my stereo on my 1995 Sport. I plan to wire the amp directly to the battery with 8 gauge power and ground wires.

I have read that the head unit should run on the same circuit as the amp but on the two cars I have installed an amp on only the ground was shared. Do I really need to rewire the head unit power wires( switched and constant) to the amp power. I have read running the power wire from the head unit can solve issues with electrical interference but it's not clear if I need to do this from the start or only if I run into issues.

If I did run the head unit power to the amp can I use an accessory breaker from the dash rather than an inline fuse on the switched power wire. That way I can switch the stereo off at the dash. What amp rating should do I use on the switched power wire?