[XeTeX] Using OTF fonts in custom letter class file

[R (Chandra) Chandrasekhar]

Dear Folks,

I have a custom letter class file in which the sender's address and 
contact details are typeset in a pre-defined font as part of a 
letterhead, independent of the font (name and size) used for the text 
body. For example, the command

\DeclareFixedFont{\viiipl}{OT1}{ppl}{m}{n}{8} % 8 pt

selects Palatino 8pt and the commands

{\viiipl\setlength{\baselineskip}{10pt plus2pt minus1pt}
\fromaddress\\}

allow the sender's address to be typeset in Palatino 8pt independently 
of the font (name and size) used in the body of the letter.

While this works in LaTeX with Type 1 fonts, I wonder if there is any 
mechanism that allows OTF fonts to be used in like fashion in a class 
file. If so, how does one go about:

1. Specifying that the class file can be run only with XeLaTeX?

2. Specifying the font (name and size) to use?

Thanks in advance.

Chandra