Reflexjs is built on top of theme-ui. So everything that is in theme-ui works with Reflexjs.

You can definitely use both at the same time. They both consume similar theme specs.

Here's an example with styles props (from Reflexjs) and sx props from theme-ui:

export function ExampleComponent() {
  return <div background="red" sx={{
    color: "body"
  }}>A div</div>
}