How to use Codeigniter with other framework?

I think you need to choose a framework. They are not generally designed to inter-operate.

I chose to move from Laravel because I was not using many of the included features and it appears slow to my clients. I ended up choosing Codeigniter due to its light weight and speed. For my needs, we really need to move a longer term supported release including patches but without breaking changes. The current reading changes and file changes can be daunting, especially if your enterprise project is several releases behind,