Considering the difference between legal and smart, you could nonetheless potentially satisfy the training requirements with just a few hours in a helicopter. FAR 61.109 specifically notes that certain requirements must be satisfied in a helicopter:
- 61.109(c)(1): "3 hours of cross-country flight training in a helicopter;"
- 61.109(c)(2): "[...] 3 hours of night flight training in a helicopter that includes" a 50nm flight and 10 takeoffs and
- 61.109(c)(3): "3 hours of flight training with an authorized instructor in a helicopter in preparation for the practical test..."
- 61.109(c)(4): "10 hours of solo flight time in a helicopter..."
Theoretically, the flight training could be all combined, resulting in 3 dual and 10 solo hours in a helicopter, the rest of the requirements being fulfilled in a different category or class of aircraft.
The FAA issued an interpretation (O'Mara, 2012) about these rules in 2012. The discussion was about weight-shift-control aircraft (61.109(j)), but the reasoning is applicable to other sections of 61.109:
In your letter you indicate that an examiner you spoke with stated that all of the aeronautical experience requirements specified in § 61.109(j), to include the 40 hours of flight time, 20 hours of flight training, and 10 hours of solo flight training must be completed in a weight-shift-control aircraft. That statement was incorrect.
...[T]he requirement for an applicant for a weight-shift-control aircraft to have 40 hours of flight time may be met by obtaining flight time in any category and class of aircraft.
Paragraphs (j)(1) through (j)(5) of § 61.109 specify the flight training that must be received by an applicant to receive a weight-shift-control aircraft rating. Flight training is defined in § 61.1 as "training, other than ground training, received from an authorized instructor in flight in an aircraft." Similar to the definition of flight time, it also does not include a limitation that the training be obtained in any particular category and class of aircraft.
That said, this is somewhat academic: in order to get a solo signoff from a CFI, a student would at least require nontrivial training time in a helicopter. I'll also cite another interpretation (Zomnir, 2010) I came across in my research, which has perhaps the most obvious statement in an FAA legal interpretation:
First, you ask whether your student is required to be proficient to the practical test standards to pass a practical exam. The answer is yes.