I'm afraid that the inaccurate-beams are the trickiest part of this. Spores, once created, are "magic" - you can't touch or edit them. Beams will target them, and you can't tell when a beam finishes firing at one.
The three-unit idea is a relatively straightforward one to do.
Direct damage from Spores is difficult - Shields will take damage from the impact, but Beams are only damaged by the Creeper payload.
Overall, it's probably just as straightforward to create a replica of each unit, since you're only working with three types. Builder17 has shown an example, but there are complete custom Spore Tower scripts out there, and the PaC maps have the manually-targetable-Spore code.