Can you only iMessage iPhone users from iPad messages?

Unless you also have an iPhone, your iPad cannot send or receive SMS or MMS messages to phone numbers - as these are Cellular Carrier features. Without an iPhone, your iPad is limited to sending/receiving messages via the Apple-proprietary Messages App.

Native iMessages are directed to Apple recipients by their registered email addresses - and not by their Cellular phone numbers. The Cellular number associated with your iPad’s Data-SIM is only used for account identification and billing purposes.

To use SMS/MMS messaging from the iPad Messages App, your iPhone must be logged-in to iCloud with the same AppleID - and message forwarding to your iPad must be enabled on the iPhone.

https://support.apple.com/en-gb/HT208386

I hope this is helpful in clarifying your iPads Messaging features.

Not unless you have cellular service on your iPad. If it's WiFi only, the you can only send iMessages.

https://support.apple.com/guide/ipad/set-up-messages-ipad70482978/ipados

You can message anyone who uses iMessage