Extracting photos out of very old iPhoto Library to import directly into Apple Photos on El Capitan
You can, but you'll lose metadata such as albums and you might end up with duplicated photos if you import both the modified and the original.

Instead, you can use the iPhoto Library Upgrader to prepare the library for import into Photos directly. This Apple tool takes your pre-'09 library and allows it to be imported into Photos. For more information on how to use the iPhoto Library Upgrader, see Using the iPhoto Library Upgrader tool.

Pre-'09 is v7.x and earlier. v8.x and later can be imported directly with Photos without this tool.