I wish to build a LED cube but have no experience in shift registers so I am building one with addressable LEDs that can draw 60mA each. I want to make an 8x8x8 cube so that would be 512 LEDs and 30.72 possible amps of current or above 150 watts of power. These WS2811 "Neopixels" are very sensitive to voltage changes and require a 1000uF capacitor to protect them according to the Neopixel guide. Is it safe to put a regular capacitor across the power to even out voltage spikes on such a high current circuit?.
No, it's not necessarily safe because capacitors appears as very low impedance when uncharged and first powered up. That initial current surge can be too high for your wiring or power supply to handle. Things can overheat, or fail, or your power supply can get stuck in a startup-loop where the protective fail-safe continually kicks in.
The simplest way to reduce this current surge is to place a NTC in series with the capacitor and load. It has high resistance when cool and lower resistance when warmer so it is basically a resistor that somewhat removes itself from a circuit as you run it.
That said, I don't see how it's any more sensitive to overvoltage than any other LED. How are you planning on powering them? More effort should be put into building a better regulator for them than slapping a massive decoupling capacitor across the rails.