They are all from former Soviet Republics and this is only an issue if your an Ethno Nationalist supporter of Navalny. They all speak Russian, and fit into russian caste and society. Tons are Ukrainians and Kazakhs and Kyrgz can be honorary 'ethnic' Russians if you must. The USSR had a referendum the non white states all wanted to remain in the Union. The Baltics wanted out and Yeltsin took Russia out inspite of its popular support to remain in. So the common myth that ethnic separtists broke up the USSR is a lie along with it failing due to poverty. Also consider that separtist regions of other former Soviet states all want to join Russia, despite having NON ethnic Russian majority populations.
Abkhazia
South ossetia
Transnistria
Luhansk and Donetsk
Etc
Well I think problem with these migrant workers from Tajikistan/Uzbekistan/Kyrgyzstan speaking/understanding around 10 words in Russian started maybe not 20 years ago, but something like 15 or so. People who works in Russia are kinda young, like 20 years old and even people who was born in 1970's not everyone can speak/understand Russian on a conversational level. If you are partially Russian or from some elite family, like doctors or Russian teacher, then yeah, good chance you can speak/understand Russian. But from what it looks like people who go to work in Russia is kinda like rednecks in their countries, so how do you think guy from villages with population around a few thousands of people are gonna learn Russian? And about fitting - nope, there is things like violence against each other, like people go to Russia and bring their hatred to other ethnicities or to people from different clans and all that stuff (I remember reading and posting here about how in Saint-Petersburg a group of males from -stan, too lazy to search, but I think it was Kyrgyzstan, was raping a Kyrgyzstani woman for hours in a hotel room and beat her and said that they will stop if she pays them -- she didn't had the money and failed to find the money, so they threw her off the balcony, she survived and then these guys tried to kidnap her from hospital). And there is other stories, like I think there is a video of Tajikistani (if I'm not wrong) naked guy fucking/raping a dog in a daylight giving 0 fucks that there is people around him, including kids. And how it looks like -- they don't care about Russian laws and fitting into caste and society and even prefer Russian prison than to return to homecountry.
And there is other stuff -- since often these people works unofficially, so they can be kicked from work at any moment and sometimes they kicked from work without any money (it ends bad sometimes, like guy refused to pay so they murdered him) and after that, without any money and food and water they turn to crime. So watching TV, report about crimes -- you can something like that (in places like Moscow), Kyrgystani citizens armed with knife robbed a shop, Tajikistani citizen raped and robbed a woman, Uzbekistani citizen arrested for selling drugs.
About them being guest workers -- look up Russian census, if I'm not mistaken (too lazy to look) in less than 10 years number of Russian citizens that identify their ethnicty as Tajik grew from around a thousand people to few thousands. And from what I understand not everyone from Tajikistan identify as Tajik, some people as Pushtun and other ethnicties. Also example - there is a fighter from Tajikistan in Bellator and if I remember it correctly he said that he can apply for Russian citizenship because his mother is Russian citizen now. Another example -- there was a report on Russian TV about ISIS and Taliban, in which they showed a few Tajikistani guys like 3 guys and 2 of them mentioned holding Russian citizenship (I think one mentioned that he went to Russia to work, earned enough money and moved to Syria to join ISIS, I can't remember it was him or other guy who mentioned that his wife, a citizen of Russia is in Russia with their kids at the moment).
Also there were things like (illegal) agencies that provide suck services -- you, as a citizen of -stan, gives them money and after that they help you marry a woman and got citizship through marriage to a Russian citizen. After that you divorce. In some articles I've seen journalists named a price (like a bribe) that these migrant workers pay and officials gives them citizenship, it's not cheap, it's more than a year of work and saving money but in the end they became Russian citizens.
Also there is stories about organizing whorehouses for *ethnicity-name* only and street advertising them (via graffiti and stickers) in their native language (it means, that there is a lot of people who demands this if people cares to organize such illegal business). There is also mma gym/gyms and mma promotion (Kyrgyzstani) in Moscow and I think there is even reports about that in English.
Also earlier this year there was a mass brawl at mma event between people from Kyrgyztan and locals, where Kyrgyzstani guys was screaming some Russophobic stuff, like death to ethnic Russians if I'm not mistaken. So there is such thing like people who hates Slavs goes to Russia to work here.
And you wrote about people from Kazakhstan -- if I'm not mistaken there is not a lot of them + isn't those who go to Russia is non-Kazakhs, but Kazakhstani citizens of Slavic/German etc descent?