I think you can define some compile time arguments. Those that controls how strings are managed.
$X+ $X- Extended Syntax, this one makes Delphi strings PChar compatible. Also allows you to use functions as procedures ignoring results
$H+ $H- Long Strings, this one turns on/off UnicodeString. Can be used locally to set strings to old Delphi behavior.
$V+ $V- This one is useful with shortstrings only as it allows you to give any sized strings as parameter when set as $V-. If you set it to $V+ then you need to pass strict string types.
This are mainly backward compatibility options. Be careful, unexpected results may happen.

Another trick you can use, is to define a local string type after detecting if Delphi or Free Pascal, and use it as you need in every parameter you pass.