Good afternoon telefrankchi,
Welcome to the Apple Support Communities!
It sounds like you are wanting to add titles to your video clips, controlling the length and the font of the title. Since you don't want to do this in the Photos app, we would suggest using iMovie or Final Cut Pro X to create a slideshow or movie. Each title has default font applied to it, but you can change that to any font on your computer.
The resources below will help you address this issue:
iMovie for Mac: Add titles
Add titles in Final Cut Pro - Final Cut Pro X User Guide
Take care.