Lambda the Ultimate

(I posted this initially to comp.lang.misc and was told to try here instead)

I'm implementing a vaguely scheme-like scripting language and I'm a
unclear on how to make multimethods work (not just within an object
system, but with all the language types). Are "types" just symbols or
do they require their own namespace?

(defun f ( (x 'int) (y 'string)) (...code))
-or-
(defun f ( (x (resolve-type int)) (y (resolve-type string)) ) (...code))

Should I combine the concept of class and and type? This seems
problematic since I want objects to be runtime-modifiable (add/remove
methods). I was also considering making predicate types as the basis
for method resolution (obj has_method x, etc..), but then how do I fit
in primitive types?

I guess I'm a bit overwhelmed with type systems and would appreciate
some direction.

Thanks,
-Alex