Mega power is ok as long as you don't turn the volume too high, the Yammies are only rated (conservatively) at 100w as a three driver system. I once saw a DJ connect the bass channel to a tweeter on the PA system by mistake, it exploded across the room when the music kicked in.
Yes, use the most powerful amp for the bass drivers and you could use the power amp ellement from the Sansui's, they don't need be the same power wise, I just prefer identical amps, remember amp sensitivity is also going to dictate how loud it sounds. Different amps also sound well, different, I just think it is easier to dovetail drivers and crossover filter elements when the gain slope, tonality and sensitivity of all the amp channels are the same. There are however, lots of people who have used different size/make of amps that have been successful, so this is just my own personnel preference.
You are trying to create the right balance, a 3000w bass amp and a 20w mid would not work as the mid amp would be too low on output, so the bass would then need the trim pot to turn it down. As a very rough guide, the bass will use about twice the power of the Mid and the tweeter a little less, but it depends on driver impedance etc so this is only the case when all the drivers are the same impedance, like they are on the NS-1000.