Was a bit unsure on which category to place this topic.

Recently, I have been reading up on different cultures/countries. Would anyone be able to provide authentic sources on what the Catholic Church says about the Russian Orthodox church? Are their baptisms/marriages valid, but are not necessarily “sacramental”? 

I personally know of someone who was born and raised in Russia and practices this faith. Any discussions with someone who is familiar with this would be so helpful!

18. Who then, considering the great extent of these labors, undertaken chiefly for the benefit of Orientals, does not trust Jesus Christ the most merciful Redeemer of men, taking pity upon the sad fate of so many, long astray from the right road, will complete what We have begun, and guide His flock into the One Fold, ruled over by the One Shepherd? A special reason for this hope is that among those nations a very great part of Revelation has been religiously preserved, sincere service is rendered to Christ Our Lord, great piety and love are shown towards His sinless Mother, and devout use made of the Sacraments. Therefore, since God in His mercy has willed that men, and especially priests, should as His instruments co-operate in the work of Redemption, what is there left to Us, Venerable Brethren, but once more to supplicate, yea to compel you not only to agree in mind and in heart with Our designs, but also to labor that the longed-for day may soon dawn, when We shall all welcome back, not only a few, but the vast majority of the Greeks, of the Slavs, of the Roumanians, and of the Eastern nations, hitherto separated, to their former communion with the Roman Church. And as we meditate upon what We have already begun to do, and what We hope to bring to perfection, so as to hasten this joyful day, it seems to Us that We may compare Ourselves to the Father of the family whom Christ Our Lord describes as calling the guests invited to His supper “that they should come, for now all things are ready” (Luke xiv, 17). Applying these words to Our own case, We earnestly entreat you, Venerable Brethren, that you add your efforts to ours, for this most important end of promoting Oriental studies. So that, after the removal of all obstacles, under the auspices of the Immaculate Virgin Mother of God, and of the Holy Fathers and Doctors of East and West, We may receive into the House of the Father those brethren and sons of Ours, so long separated from Us, but once more united in bonds of a charity based upon the solid foundation of truth and the full profession of the Christian religion.

Not only are the eparchies (dioceses) of the Orthodox schismatics true particular churches (not meant here in sense of the True Church) with valid sacraments, Catholics may avail themselves to certain Orthodox sacraments (Eucharist, Unction, Penance) in an emergency.

we don't know if their orders have the correct intention due to communism

Completely false.  So long as proper form is used (and it was by the Moscow patriarchal church throughout the hrars of communism), the Church always presumes validity of a sacrament. One would have to prove invalidity which is very hard to do since invalidating a sacrament is very difficult without altering form, matter, or minister.

I've heard some people say that the validity of priest from the soviet block is not absolutely certain since, they say, KGB agents (who wore fake beards for a time ) replaced some priests at some point.
But invalidity because of communism? It's the first time I hear that.

Completely false.  So long as proper form is used (and it was by the Moscow patriarchal church throughout the hrars of communism), the Church always presumes validity of a sacrament. One would have to prove invalidity which is very hard to do since invalidating a sacrament is very difficult without altering form, matter, or minister.

And what of the primacy oath to the Communist state each priest/prelate had to take or face a long train ride to Siberia?