Is the 6,000W rating real RMS power?

No. 6,000W is the max (peak) figure. Continuous RMS is 1,800W at 1 ohm, 1,000W at 2 ohms and 600W at 4 ohms. Size your subs and wiring around the RMS numbers.

What subwoofer load can it drive?

It is stable down to 1 ohm, so a single sub or a pair wired to a 1-ohm final impedance works. At 2 and 4 ohms it makes less power, so match the sub's impedance to your goals.

Will my stock electrical system support it?

At 1 ohm the amp pulls high current. Expect to need upgraded power and ground wiring and likely a battery or alternator upgrade to avoid voltage drop and clipping.

How does it compare with a name-brand monoblock?

It offers more rated RMS power per dollar than premium brands, but those rivals typically add better protection circuits, cleaner boards and stronger warranty support. Choose this if raw output matters most.