You CAN fly a Decathlon or a Citabria at ANY level of competition in IMAC. At least around these parts you can. A Decathlon and a Citabria are both aerobatic trainers that have been flown at full sacale competitions- they are legal to fly at IMAC in any class.

Generally, you can enter and fly ANY plane that is scale and has been flown at an IAC event.

Your not going to WIN, because these planes don't fly near as well as a Cap, Extra, Yak...etc.. People don't generally fly the Decathlon or Citabria because they aren't capable of flying as precise or smooth as the other more popular aerobatic thoroughbreds.

I don't think you can enter a Cub, Beaver, or Cessna 150 in an IMAC event. Simply because these arent aerobatic planes and have never been used to compete in an IAC event. You could enter one of these planes in the basic class, because that class doesn't require a scale aerobatic plane. It's a "get your feet wet" class and should welcome anyone with a Kadet LT-40 if they want to try it out.

I'd love to enter a giant scale Decathlon in the basic class and see how I could do. Probably get my butt smoked, but it'd still be neat to see a Decathlon doing all that stuff in a competative environment.