What is a good SC and MC to focus on monastic training passive? How is it work anyway in this game - flat on top of base like in PoE 1 so crits and grazes are nonexistent or is it more of proper bonus so something like cipher whip is fully applied to it?

It well described in PoE Gamefaq from Thelee. Easy to googlelize.

It's good with any class, except monk ofc.

It's very different than Poe1 version, it basically works like a summoned weapon , however PL affects it much more than it does summoned weapons (as Elric said the gamefaq has the proper numbers), so it easily outscales those in the later stages of the game and on top of that is much more powerful than most actual weapons you find early game.

Personally I usually pick the talent on literally any class unless I'm super starved on passives, because then you can be quite flexible with your pen, especially if your main weapon are the oh so powerful bleeding cut battle axes.

The only reason to ever use summoned weapons are if you either want to use an utility one like Spirit Lance (very amazing with ciphers) or you have a multiclass that simply can't pick monastic like chanter/wizard for example.