It's worth noting that unlike the president of the USA, the president of the Russian federation doesn't actually have the authority to order a nuclear first strike. He has autonomy to initiate a response to an incoming nuclear attack, but that's it.
It's a holdover from Soviet times. If he wanted to use a tactical nuke in response to a conventional attack (I.E invasion of Crimea) he would have to assemble the required ministers (or at least a quorum) and put it to a vote.
I'd be very surprised if in reality that was anything more than a formality, though how that works for tactical vs strategic use I dunno as well.