I have iphone se (about 6 years old). Can I use it with USB C adaptor/lightning cable?

Yes. They're supposed to be compatible with anything that has a Lightning input port - all the way back to the iPhone 5. Apple's compatibility list includes the 1st generation iPhone SE

Buy USB-C to Lightning Cable (2 m) - Apple

iPhone Models

• iPhone 13 Pro
• iPhone 13 Pro Max
• iPhone 13 mini
• iPhone 13
• iPhone 12 Pro
• iPhone 12 Pro Max
• iPhone 12 mini
• iPhone 12
• iPhone 11 Pro
• iPhone 11 Pro Max
• iPhone 11
• iPhone SE (2nd generation)
• iPhone XS
• iPhone XS Max
• iPhone XR
• iPhone X
• iPhone 8
• iPhone 8 Plus
• iPhone 7
• iPhone 7 Plus
• iPhone 6s
• iPhone 6s Plus
• iPhone 6
• iPhone 6 Plus
• iPhone SE (1st generation)
• iPhone 5s
• iPhone 5c
• iPhone 5

Any USB-C power adapter will have 5V available, and is supposed to default to 5V first before negotiating operation at a higher voltage if compatible. The electronics in the cable are supposed to know the requirements for the device it's dealing with, although I'm not sure how non-Apple devices with Lightning ports (like some USB power banks) are handled. As long as it's an Apple OEM or MFi cable, you should be fine.

Yes; you can uses any iPhone that has a Lightning connection with either a Lightning to USB cable or a Lightning to USB-C cable.