Thank you for your post.
Bento Templates cannot be imported into FileMaker Pro. Only the data can be transferred. You would have to recreate the Bento layouts (Forms) in FileMaker Pro.
Bento 3 (and Bento 2) should be able to handle 6000 contacts. I have a couple of Bento Libraries with more than 10,000 records for testing purposes.
Knowledge Base Article #7411 contains the Technical Specifications for Bento 3: