Taxing 1, 6 builders is obvious. Is a basher also nessecary or can it be done without bashers?
Taxing 2, I've attached two replays that between them, eliminate every skill except for one builder.
Taxing 3 is however many builders is needed to get a path to the exit (unless you can do it in fewer with sliding, maybe?). You can create a holding area without builders by using the dig-on-same-spot trick, and using a climber who then digs and bashes to release the crowd. The level is possible with builders only, so no non-builder skill is essential.
Taxing 4, a normal solution can be done with either miners, or diggers and bashers, to get the two sides together, and apart from that, only builders are needed. So, what's the fewest amount of builders it can be done with? It seems to be... 3, by using almost every other skill, and TONS of glitch abuse (starts with blockers pushing lemmings into walls, then there's steel glitches, lemmings walking through blockers by coming up under a blocker on an edge, and concludes with a good old slide straight up to the exit).
Taxing 5, I'll leave this to someone else.