Generally the former (I am discussing the Soviet era, BTW). The Latvians were so disgusted by this practice that their post-independence naturalization law specifically excludes from eligibility anyone who settled in Latvia after being demobilized from the Soviet military (and their families).
Looking at the CIA world factbook, here’s the percentage of Russians in each of the other former republics (it’s not the most up to date info, but it gives you the idea):
Belarus: 11.4%
Ukraine: 17.3%
Moldova: 5.8%
Estonia: 25.6%
Latvia: 29.6%
Lithuania: 6.3%
Armenia: 0.5%
Azerbaijan: 1.8%
Georgia: : 1.5%
Kazakhstan: 30%
Uzbekistan:5.5%
Kyrghyzstan:12.5%
Turkmenistan: 4%
Tajikstan: 1.1%
Well, you can see why the Letts got all het up!
Those are the current numbers, but the numbers were much greater before 1991 - ethnic Russians have been getting the hell out of many non-Russian areas of the former USSR at a rapid clip since its breakup. I will try to post more cites later (I have a bunch of stuff bookmarked at home).
Here’s an interesting article summarizing migration trends in the FSU over the past 20 years. More later.
I just don’t see it. If we work forward from the 15th century, we first have a few centuries where ‘Georgia’ was merely a region, with lots of separate kingdoms and principalities. Then we have those principalities getting gobbled up by the Tsar. Then after a 3-year bout of Georgian independence where Georgia governs South Ossetia, it gets gobbled up into the USSR. Then after a year where Georgia is independent and governs South Ossetia, South Ossetia gains the right to be autonomous within the bounds of Georgia.
So I see four years in the past five or six centuries where South Ossetia was under Georgian governance in a meaningful sense.
Eva, has there ever been an Ossetia?
You mean as a separate kingdom or somesuch? I’m no historian - you want Tamerlane or someone else who is a much bigger history geek than I am.
There was an Alan state in the north Caucusus during the middle ages.
I just looked all that up on Wiki, and it’s mind-blowing. Georgian and Ossetian aren’t even in the same language family. In fact, the Georgian languages aren’t in a family with anything. Plus the Russians, right next to them, are a third group, totally unrelated to Georgian, and only barely related to Ossetian!
You’ve got to wonder about the stubbornness of some peoples who have managed to live that closely together and still haven’t merged languages or cultures. That takes some serious bull-headedness. 
ETA: Of course, everybody there speaks at least three languages, so it’s not like they’re totally closed to one another, but still…
Yeah, if you want a headache, take a look at this linguistic map of the Caucasus.
And as for the ethnic Ossetians, they have probably never numbered more than a couple hundred thousand, so even if they ever had a kingdom, it wasn’t exactly huge.
P.S. Serious bull-headedness: well, having had one Caucasian ex-boyfriend, I am perhaps biased on that point. 
But also keep in mind how mountainous the area is (and how largely rural, especially until recently). It’s not exactly easy to get around there.
And Poland has just signed on to the missile defense shield.
Or north of the Caucasus. The Georgian dynasty was actually an Alan dynasty in a sense after the 12th century, as all Georgian kings down to 1801 in the later “Bagratid” dynasty were descended from David Soslan, the Alan/Ossetian consort of the Bagratid Queen Tamar ( r. 1184-1213 ). The dynasty just continued to use the dynastic name of Tamar’s house.
As the good Captain noted, it was the Mongols that put an end to that state and caused the Ossetian migration into the mountains.
RTFirefly: I’m going to somewhat side with Captain Amazing on this. No, the Kingdom of Kartli was not identical to modern Georgia. But it WAS a Georgian kingdom, ruled by a branch of that same ancient Bagratid dynasty. The south Ossetians was always under some sort of Georgian suzerainty in that sense, pre-20th century. Never was that area independent - as Eva notes, it’s a mighty small ethnic group and the Georgians around them considerably outnumbered them and were politically dominant.
These people aren’t the savages tagos was trying to claim, but they are certifiable! On the other hand, Welsh is still alive and well, so what can I say?
You know that territory far better than I do. Thanks for clarifying things.
And Kosovo was part of Serbia, Israel part of Ottoman Palestine and the sun never set on the British Empire. Past history and irrelevant.
South Ossetia has never been governed by an independent Georgia in the latter half of this century. They have at best been an autonomous province not recognising georgian authority and successfully fought a civil war to make the point.
So basically Georgia’s claim is basically ‘it belongs to us because Georgian Uncle Joe Stalin said so.’
Forgive the hell out of the South Ossetians for not thinking that a very sound claim.
Wholesale population transfer to dilute ethnicity as a potential basis for opposing russian communist control of the USSR has always been soviet policy. It has left a ‘legacy’ of Russian ethnic cuckoo’s all over the map including the baltic republics.
But as Eva pointed out with cites, a lot of those cuckoos are flying back to the home nest, now that they’re discovering that the sparrows aren’t terribly welcoming without the might of the Soviet Army backing them up. I imagine over time the rest will largely assimilate; they’ll have to unless Russia decides to recreate the FSU (Now without Communism!).
Many, but not all, particularly if they are married to members of the local titular ethnic group and/or speak the local language.
Sorry, I fell asleep last night before managing to dig up some of my census statistical links, but in many cases the non-indigenous Slavs fled back to Russia in cases like those of the Baltics or Central Asia, in which most never learned the local language or intermarried or otherwise assimilated into local culture (you will just about never find an ethnic Russian who speaks Uzbek or has converted to Islam, for example).
(And darn the Russian state statistical agency, Goskomstat, for apparently making detailed census statistics available only by subscription. When I was doing initial thesis research, they made much more stuff available for free, but half the time when I’d go back to verify stuff, it was no longer available. So a lot of my old bookmarks don’t work anymore, anyway. Darn ex-communists! And there were those who never thought Soviets could learn capitalism…)