Moving photos from iPhoto to Photo on separate Macbooks

Copy the Photos Library to an external volume that is formatted MacOS Extended (Journaled) and has not been used for Time Machine backups. Enable the "Ignore Ownership on this volume" flag for the external volume. This will prevent permission and file ownership issues. The volume you are using to copy the library should be prepared as described here: Move your Photos library to save space on your Mac - Apple Support

Don't do any of that.

Simply copy the iPhoto Library from the old machine to the new one. Then open that with Photos. It does the rest.

In relation to exporting mp one can tell you the best 'kind' to use. It depends on what you want. The original gives you exactly that: the image as copied from your camera. Current gives you the edited version, if you've edited. Jpeg, TIFF and PNG are different file formats. Tiff files are not compressed so huge. Jpegs are a good trade off between size and quality. PNG are almost never used these days.

iPhoto does allow you to export video, btw, just use the kind as Original.

How have you tried to open it?

Have you tried holding the option key down while starting photos, and then selecting the copied library?

What happens when you do?