You can use functional and class components in the same project. Prior to hooks being introduced, this was nearly a requirement since class components were the only ones that could hold component state and hook into life cycles.
Since hooks have been introduced and largely removed the differences in terms of what can be accomplished with class and function components (although there are still a few holdouts), it is generally recommended to use function components and hooks.
So to summarize, there are no technical problems with using class and functional components together, but it is generally recommended for new development to use function components.