Russian Mafia takes recruits, kills uppity upstarts.
Russian mobnicks & convicts do not learn to fight a a group, but as suicide soldiers. Individual survival is all that matters. Dead are left to rot, and wounded are often shot.
US Military is NOTHING like the Russian army - even those doing hard time are ‘nicer’.
Sometimes you overdo your impulse to put everything into a perfectly formed box, with reality being a precise and exact fit to your imagination and opinions.
We don’t know the precise level of training every individual receives, but we can assume that out of those thousands, some will be made professional mercenaries and some will be pulled out for officers and NCOs.
A giant mercenary organization is not going to get a pool of 10s of thousands of hard men and not find a lot of good individuals from that pool.
As far as the Russian mafia not having any trouble with them, my bet is that they will, things change, people change, organizations change, leadership is fought for and changes.