This is the best advice I have ever seen and used.
Infantry divisions: obviously the main component is infantry. The second most important is artillery. In principle, the more artillery the better. However frontage limits the number of guns that can engage at once (in open terrain frontage is wider, in mountains, swamps, hills or woods smaller). From a certain point onwards more guns don't help. Moreover a division needs a sufficient number of infantry to absorb damage. During battle resolution infantry and cavalry take damage first, artillery last. It is not unusual to see the entire infantry component of a division wiped out while the artillery escapes without a scratch. Therefore I would advocate for 2 artilleries per division as minimum and 7 as maximum, all the while observing a ratio between infantry and artillery of at least 3:1.
- tanks: not more than one per division, anything more is a waste, pure tank divisions are stupidity (the Germans get one in the Drang scenario for example, split it up!). The tank doctrine in RUS is that of WW I rather than WW II. Tanks have the armoured support ability which grants infantry within the same division an initiative bonus (higher chance to fire first) and improves their combat effectiveness by +10%. Moreover there is a bug that seems to change the movement rules for infantry divisions if they contain a tank element. Basically such a division moves about as fast as cavalry. Never leave tanks on their own - it is a waste of the armoured support ability; never put them in a cavalry division since the ability only affects infantry.
- armoured cars: basically these are a poor man's tanks in RUS. The only important difference is that there isn't a bug affecting the movement of the entire division.
- engineers: I sometimes put them within a division but they are fairly soft and tend to suffer from it. The important thing is to have one per stack (entrenchments are built a lot faster), my impression is that it is more beneficial to leave them outside the divisional structure.
- cavalry: having a single element of cavalry per division improves its detection value (it will deliver more precise intelligence on opposing forces)
- tachankas: 1 per division (not more) is useful. They provide fire support (+1 initiative bonus). Common sense seems to indicate that this weapon was invented to support cavalry but they are just as useful combined with infantry.
- specialist infantry: mountain infantry for the Siberians, Cheka Line infantry for the Reds. Their special abilities affect the whole division. More than one per division is a waste.
Cavalry divisions: Everything that could slow down such a division has no place in it. This leaves three incrediants: cavalry, horse artillery and tachankas (1 per division, more is a waste).
Please find further discussion at http://forum.paradoxplaza.com/forum/showthread.php?571687-Who-put-the-stranded-Admiral-in-charge-Siberian-White-Short-Campaign-PBEM&p=13161471&viewfull=1#post13161471