While the Russians were indeed in Crimea, and the Ukraine of today is a breadbasket, at this time they didn't have the right ploughs for the heavy Eastern European soil. So they'd essentially acquire a big empty plain which isn't too useful.
I guess, you've meant the
Romans in the Crimea? Because the Russians didn't exist at the time. Regarding heavy ploughs... I don't know exactly, but in the time of Kievan Rus' the Slavs tilled the steppe soil with relatively light horse-driven plough (
sokha), and it worked. Of course, they weren't regular exporters (though odd shipments to Constantinople occurred), but famine wasn't frequent, too. Herodotus mentions Scythian export of grain to Athens as early as 5th century B.C. So, I think, technologically colonization of the Ukraine could be possible. Was it possible militarily or socially - that is another question.
The Romans subdued or expelled nomadic tribes in Northern Arabia, Syria and Africa, allowing for the colonization to proceed (sometimes even to achieve prosperity - as in the Roman provinces of Africa and Numidia). Great Eurasian steppe is wetter (and colder, alas) than northern fringes of Sahara, so potential for the agriculture is at least comparable
Probably go something like Darius' invasion of Scythia: Scythians refuse to fight, and retreat, Romans get bored and go home.
Maybe, and maybe not. The Roman Army could defeat the nomadic cavalry. It did so many times in Africa, Syria and Arabia. And Scythians (as well as Sarmatians, who were paramount tribal union of the steppes in the Roman age) couldn't flee indefinitely. Each tribe, and each clan within the tribe had its own winter and summer pastures. It couldn't use neighbor's grass without agreement, and to achieve such agreement with all tribes concerned would be very hard task. Moreover, some steppe inhabitants were full-time or part-time agriculturalists (slaves or vassals of the nomadic confederations, as well as poorer members of these confederation themselves). For the peasant to flee his plot meant to face death from starvation, so the Romans would find some population in the conquered territories, without doubt.
Roman invasion to Russia is very implausible. Roman should conquer the entire Germany, Poland, and the Baltics in order to conquer Russia successfully.
But there was another route of the invasion, proposed in this thread - from the Black Sea ports. Lower Dnieper and Dniester could be good means of river communications.