I've spoken with several freelancers and agreed with them on a fixed price. Is it ok to tell all of them to start building project's initial prototype and at the end hire the one that we like the most?
You'll have to hire them all... but just for the small prototype project. Then you can hire the one you choose again for the main job.
- Box, Logan's Run (1976)
Smart clients save money by paying multiple freelancers to do initial work before deciding upon a final candidate.
If you don't pay for some work that you throw away, I guarantee you are going to waste money in the long term.
You can hire them all using hourly contracts and let them work on prototypes for a few hours, carefully reviewing their work, and cancelling contracts with those whose work you don't like.
You could also pay separate fixed-price contracts for creating prototype only.
Or you can hire only one person to do anything, without actually knowing if that person is the best.