Hello, I do a lot of timelapse work myself, feel free to ask me anything. I use a 5D or 6D with Magic Lantern software loaded on the camera. Not completely necessary but can do a bit more than a simple external intervalometer (which also works well). The timelapse is going to be a series of raw images in whatever resolution the camera shoots at, 5.7k on the 5Diii for instance. It should be pretty easy to match up with the Alexa footage in the grade.
The post processing can be a bit tricky depending on what you are shooting. If you are doing shorter lapses without drastically changing light than post is pretty simple, it only gets complicated when the light changes (such as sunrise or sunset, or passing clouds) and you have to do special deflicker processing to the image.