Should I take a catamaran course before chartering a catamaran?

Strongly advisable, and more so the more monohull experience you have — because much of what you have learned will actively mislead you.

A catamaran manoeuvres on differential thrust from two widely spaced engines, which makes it far more controllable in a marina than a monohull once you understand it, and far more alarming until you do. It has high windage and a wide beam, so it moves sideways in a crosswind in a way that surprises people, and berth selection matters more.

The bigger issue is feedback. A monohull tells you it is overpressed by heeling. A catamaran does not heel meaningfully, so the warning you have relied on for years simply is not there. Reefing decisions have to be made from wind speed and sail load rather than from feel, and getting that wrong on a cruising cat is genuinely dangerous.

Add bridge-deck clearance, different anchoring behaviour, higher loads on every line, and helm stations that are often elevated and offset. A day or two of multihull-specific training before a charter is cheap compared with the excess on a damaged catamaran.