How about this.
Scenario 1: Meeting Engagements- 10 Tacticals (in two combat squads) vs. 20 Boyz(in 2 mobz of 10)
Scenario 2: Holed up at HQ- 10 Tacticals in one squad with a captain vs 20 boyz in 2 mobs. A unit of 5 Nobz. Tacticals are hanging out in a building for x turns.(random game length maybe?) This one teaches the value of a hard unit as well as hordes, multiple wounds, and ICs
Scenario 3: Da Head of Da Horde: 10 tacticals in 2 combat squads,captain 5 terminators vs. 20 boyz in 2 mobz 5 nobz warboss. Both sides are gunning for the others HQ.
Scenario 4: Iron Fury: The entire contents of black reach in an annilation mission.
Scenario 5: mobilize: The contents of Black reach: And the orks get a trukk and the marines get a rhino. Teaches dedicated transports.
Scenario 6: The Assault on Ghospora Hive: In addition to what the players already have(black reach and the dedicated transports) They each have a battleforce.
Congradulations you now have a decently sized army.
Edit and add story as seen fit.