Does one RK-8 replace MAGROCKS 2.1?

No. MAGROCKS is a 2.1 rock kit. RK-8-GY is one 8-inch rock speaker. Buy the count the bed needs, or step up to UltraScape.

Can I mix rocks and AcoustiScape?

Yes, when part of the bed should look like stone and the rest can take a stake. Keep the amp and impedance consistent.

MAGROCKS vs TruAudio RK-8-GY rock speaker

Do not treat a rock speaker as a full UltraScape landscape system. Use RK-8-GY when the hardware must look like a garden rock. Use UltraScape when you need a burial-sub-plus-satellite soundstage.