I've never spoken to a self-declared solipsist before, though of course we all act like solipsists to a degree :-). Anyway, I will assume that solipsism is false for the rest of this post, that's another question.

It's amazing how many philosophical debates end up at the question of universals that you've just alluded to.

My own position, very briefly, is that when we predicate 'duck' (as in "this is a duck") of a given thing, we are describing reality, not just conveniently labeling some part of it in our own minds. If 'duck' is merely a label that we apply to something, then anything we predicate of 'duck' is merely something we predicate of our own mental categories. But this isn't so: the sentence 'ducks quack' refers to something real, not just our thoughts. But at the same time, the sentence is not referring to Duck A or Duck B, but to ducks in general. From this, it seems to follow that some general 'ducky-ness' must have a kind of existence (otherwise how could we predicate of it?), and that this 'ducky-ness' must be shared by everything that is a duck (otherwise, by what is it a duck?).

In the opposite scenario that you've described, all predication would be limited to our thoughts. Someone could say "ducks quack", and someone else could say "ducks never quack", and both would be right, because both would merely be describing their own thoughts. Obviously, all reason, science, possibility of communication, etc, is finished at this point :-)

Of course our labels can be wrong. Someone could mistake a swan for a duck. Also, there is infinite variation from duck to duck, so the 'ducky-ness' of each duck in no way tells us everything about that duck. Duck A and B are unique individuals. Also, the 'ducky-ness' only ever exists in a given duck; it's not like it has some independent ethereal existence.