Can I use Samsung Super Fast Charger (25W) to charge my new iPhone 15 (not pro)?

You can use the larger power adapter with no issues. The iPhone will not draw more power than it is designed to no matter how large the power adapter.

Also see the following from Fast charge your iPhone - Apple Support

To fast charge, you need an iPhone 15 model with a USB-C cable like the one that came in the box or an earlier iPhone model with a USB-C to Lightning cable and one of these adapters:

• Apple 18W,* 20W, 29W, 30W, 35W, 61W, 67W, 87W, or 96W, or 140W USB-C Power Adapter
• A comparable third-party USB-C power adapter that supports USB Power Delivery (USB-PD)