I think a flat production time would do better, but that would require engine changes.
And what do I mean, you ask. Well, basically it means that, no matter the amount of workers put into it, each recipe will require X amount of time to pass before it finishes. Any remaining work hours put into it will either be wasted or put into the next iteration of the project and such time will pass independently of the amount of workers put into it. But will only start ticking once the amount of work hours spent are greater than 0. Add some proportion maths into account and you can have a dynamic idle time cap for the project, meaning that enough work was done for the timer to start ticking, add a field that accepts either percentile (below or equal to 100%) or absolute values of work needed to start the timer and we have something somewhat decent for modders to use.
I think this is over-engineered, I'd just make it so the end product goes into "Transfers" with a set number days to arrive. Just like for some transformations.
Nevertheless, a change in code would still be required.
If you are going to use the facility idea, though, make sure they require more than one STR_GIANT_RAT_TERRORIST to be made.
I thought the facility shouldn't need any creatures just to be built. But the actual breeeding would.
But your idea may actually be better. You need, say, 10 x STR_GIANT_RAT_TERRORIST to start the rat colony. After that, you don't even need any rats to make more, because it is assumed that you already have a growing rat swarm, so recruiting new rats becomes a matter of selecting some promising individuals and running them through basic orientation. And no new code required.
But I like my things furry.
OK XD
For now, let's focus on rats... This is all purely conceptual at this stage.
EDIT: Made the map.
See attached.
EDIT 2: Basescape.
EDIT 3: It's in the main mod! Thank you for this fun idea, Thermite. Of course, you'll be credited.