I've made a small test map for harvesting critters for resources (like AoE and other rts's). It is a decent implementation in this map but I am working on replicated AoE exactly (Kill unit, harvest corpse all with a gather command hopefully).

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I have an idea on how you can implement your idea.

-A duplicate unit of the standard mineral field, "Corpse" is created; it's pretty much the same as a Mineral Field except for model, in that it looks like a corpse.
-A duplicate behaviour based on how Zerg buildings spawn Broodlings is created, except that instead of Broodlings, a Corpse is created; call it "Spawn Corpse".
-Give the "gatherable" critter units the "Spawn Corpse" behaviour.

This way, workers go out, are manually issued the Attack order to kill a critter, and then are ordered to gather from the Corpse.

I think that the downside of this is how many times the player has to order the worker, but I think it could still work. A workaround is that you can trigger the creation of the Corpse, so that you'll be able to pass some functions through and get the Killing Unit (SCV) to harvest Last Created Unit (Critter) from Event Unit Dies and Conditions (Dying Unit = Critter).

(IMHO, I think either of the ways I suggested are better alternatives, since what you've got so far is kinda... eleeuueugh. It'd be interesting, but it looks weird and it's weird that the SCVs just go out and auto-find more Critters...)

I could probably make a test map for both alternative routes for an AoE critter-gather effect, although my critters would be landed Vikings and when they die, they would drop scrap piles of metal or something, since I think that makes more sense...

But if you ever played the real AoE or any other similar game the villagers would auto gather anyway.