Flex Accounting: new business unit versus new company

We bought a company and are integrating them with our existing system. (We are in the testing phase). We already have a multitude of companies set up and wanted to avoid setting up yet another. So let's say this purchased company is being merged into our existing Co 100. Let's say we have some parts that have GL Class code IN10 set up on them. These book to 100100.40000 for revenue. For this new entity, we want to book to 200100.40000. Can we implement Flex Accounting just for a few GL Class codes we would like to redirect? Or is