Maybe contact the Flight Safety Foundation? They may have data.

There is no issue flying 2 types. Its pretty normal at most larger departments. Schools are usually alternated every 6 months. Its very difficult to schedule trips if guys are not dual qualified once you reach 3-5+ aircraft and 2 types. Nobody was allowed to fly 3 types, even if they wanted to and even if they were similar (3 types of Falcons for example)

If there was a shred of proof that it increased accident/incident data it wouldn't be the industry norm. The rest concerning proficiency and comfort is all opinion. Is it easier flying one type? you bet....but prove its safer? good luck.