NO, Russians and Soviets are NOT interchangeable terms. Russians did make up the majority of the population of the USSR, but there were also numerous other nationalities in the union and to reduce them all to one term would be ignorant and insulting.
In relation to Estonia and Estonians - you could call Estonians who lived in the USSR Soviets, because they lived in a Soviet Republic and were part of the Soviet Union, although it's better that you don't, because as PlainPat said, Estonia was annexed by the USSR (= didn't join the union voluntarily) and for example, i know my parents who are Estonians and lived in the USSR, would be quite offended.
Now on to the term Russian - no, absolutely not. Russian is a nationality and so is Estonian, so again, no, you can't call Estonians Russians (and due to the painful past including the Soviet occupation, mass deportations of Estonians to Siberian work camps and a general aim to Russificate Estonia and make Estonian culture and language obsolete, it's understandable that most Estonians would find that highly offensive, even more than the term Soviet).
HOWEVER, there is a Russian minority in Estonia and many of them do call consider themselves Russians (some prefer the term Russian-Estonians, some Russian-speaking Estonians etc), so calling the Russian population of Estonia Russians would not be offensive.
Me, my parents