In regards to the legal classification of Holodomor as genocide, I would strongly recommend the legal analysis by Yevhen Zakharov of the Khakiv Human Rights Protection group:
Opinion: Legal classification of Holodomor 1932-1933 in Ukraine and in Kuban as a crime against humanity and genocide
The basic fact is that due to the rolling back of free-maket NEP policies of the 1920s and forced mass-collectivization, shortages and famines broke out all over the territory of the former Soviet Union in the 1930s. However, only in Ukraine did it take on the nature of a crime, because every shred of food was requisitioned from every village house, and Ukraine's border was cordoned off to prevent Ukrainians from leaving Ukraine's territory in search of food.
As far as targeting of ethnic Ukrainians is concerned, it's very simple. While there were many ethnic Russians in Eastern Ukraine (who largely came there in late 19th, early 20th century because of the rapid industrialization of the region), they were overwhelmingly workers (not peasants) and lived overwhelmingly in the cities, which were not targeted by Stalin's policies. Hence, Ukrainians were targeted almost exclusively by definition.