Which factor, self-interest or idealism, was more important in driving american foreign policy in the years 1895-1920?

This time period really should be differentiated.  The early part of the period was dominated by self-interest while the end of the period had aspects of idealism about it.  Overall, I would argue that self-interest predominated, but it would be much clearer if we separated this period into the WWI Era on the one hand and the time before it on the other.

The early part of this time period is dominated by the Spanish-American War and the Roosevelt Corollary to the Monroe Doctrine.  These things showed the US doing things like occupying ore even taking other countries (Nicaragua, the Philippines) to promote its own interests.

The WWI era is dominated by Woodrow Wilson and the Fourteen Points.  Wilson's goal was to create a world order that would be conducive to peace.  This is a much more idealistic motive than was present in the early 1900s.

Overall, I would argue that the self-interest predominated, but that idealism came more to the fore at the end of the period.

what was the significance of the plessy vs ferguson ruling and jim crow laws

answer the impact. plessy v. ferguson allowed 'separate but equal, also known as segregation, to become law in the united states