Per the last bullet of 4.2.4 see rule 13.1.5 for stacking with Russian Redoubt (artillery) units in the Poltava game. Per that rule the Russian player can stack 1 infantry unit with a Redoubt.
The Russian artillery units represent Redoubts (and that is what is printed on the counters). These units have an intrinsic infantry garrison as well as artillery crew.
I was thinking of the Narva game. Since the main rules specify that you can stack Swedish arty, but is silent with the Russians, I am assuming that Russians cannot stack.