"Is adding an unsigned operand to a signed one legal?"

Are you talking 'C' or assembler?

In 'C', yes of course it's legal.

The 8051 itself has no specific support for signed numbers, so in assembler you will have to do it all "manually"
ie, you need to decide whether the value 0xFF represents 255 or -1...

BTW: When starting a new question, it's best to start a new Thread!