I've reused film cassettes off and on since the 1980s. Back in the day, Ilford's cassette's could be easily disassembled to be reused.

The best tool you can have is a film-leader retriever. That way, if you rewind your film all the way into the cassette (either by accident or that's how your camera rewinds film), you can retrieve it without having to pry open the cassette.

What you want to do is to cut the film, leaving the last and a half protruding from the film cassette. That will give you enough film to tape the new film to the old film when you're bulk loading.

I also prefer reloadable cassettes, but I've used this method many, many times when I wasn't able to get reloadable ones.

Also, get yourself either a Sharpie to mark your film or a label maker, so you know what film you've put into the cassette.

Check the felt seal on the film cassette for debris or film chips before bulk loading.