This is a really difficult question to answer without some clarification. Primarily, which definition of genocide are you using? And do you have particular instances of ethnic cleansing in mind with regard to your second question? Because it assumes a particular answer to the first half of your question.
I'll offer my multi-part comment here as something helpful to read through - it's addressing a slightly larger question as to "how intentional were mass deaths in the USSR". The big examples that I cite (and that tend to get cited) as possible cases of genocide are: the 1932-1933 Famine in Ukraine, the 1931-1934 Famine in Kazakhstan, the more localized famines in that same period in parts of Russia (mostly the Kuban, lower Volga, and North Caucasus regions), certain targeted mass killings like the 1937-1938 Polish Operation, and the wartime/postwar deportations (and accompanying mass deaths) of national minorities like Crimean Tatars andChechens.
From that whole discussion I'll specifically pick out Michael Ellman's "Stalin and the Soviet Famine of 1932-1933 Revisited" (which can be read here), particularly his conclusions. He argues that the legal definition of genocide under the United Nations Convention on Genocide is very strict (an explicit intent is part of the definition). By these standards, there is legal debate as to whether any of the instances above count (he argues that Ukraine and the Polish operation might, and that the Kazakh case wouldn't because it was a "negligent genocide", ie there wasn't explicit intent, but the consequences were effectively genocidal). The other cases he thinks would not qualify, but would qualify under broader definitions of genocide. However he also makes the point that under those broader definitions, a number of other countries (including the US) would also be guilty of genocide. I would also add that the UN definition was adopted in part by Soviet participation, so it should not be surprising that the language is of unclear applicability to the USSR itself, but that Raphael Lemkin, who originally coined the term genocide, explicitly applied it to the Ukrainian case.