Enlisting Active Duty for 35P in a month. Will the Army consider assigning me to my fluent foreign language?

You will almost certainly be assigned Russian. You'll need to take the DLPT Defense Language Proficiency Test to determine just how fluent you are. In your MOS you'll always need to score at least a 2/2, that's about the proficiency of a middle school kid. That's not a 2 out of 2, that's a 2 reading and 2 writing. 3/3 or higher is where you want to be as a native speaker, which is about a high school level. The Army may still send you to DLI for advanced language training

Depends on the language need of the Army but I would think they can use you as a Russian linguist right away since you’re qualified. After all, Russia is still a “Threat”! Всё хорошего!

You need to take the DLPT; check with your recruiter about coordinating with MEPS. It is not enough that you know Russian, but to what degree and how fluent your English is as well. Russian is a priority language, but I would recommend asking for another language anyways to get the opportunity to obtain even more valuable skills (think of the life value in fluency between Russian, English, and Persian-Farsi, for example). If you have a sufficient passing score in Russian and they allow you to attend another language, you have a safety net you can fall back on to continue into the MOS 35P.

The minimum requirement for 35P is 2/2/1+ (2 each in Listening and Reading and a 1+ in Speaking as determined by the Interagency Language Roundtables as opposed to the collegiate system’s ACTFL). However, that will be increasing to 2+/2+/1+