As long as it is not heating, the worms should do fine.

If you are hoping to 'harvest' vermicompost from your worm farm, you may want to plan for that at the start. I didn't -- just kept adding worms to an out of the way pile of compost and horse manure --- threw together sides as the pile grew, 'till was about to collapse.... then had to 'work backward' toward a better begining. **WHAT A ROYAL PAIN!** Also, hope the spot you chose for for worm farm is in the shade. I don't have to worry about moles, but keeping the squirm-herd moist and cool is constant here in the arid southwest. If you have to worry about cold winters, you might find useful hints at redwormcomposting.com And to **FINALLY** answer your question, I think that a combo of grass clippings and horse manure will be primo for starting an outdoor vermiary.[is that a word?] and BTW - now that my vermi farm has real sides and good shade, I am enjoying finding new treats for the herd.[pretty much the usual things that bin-raised worms like, but I don't have to worry about too much moisture... But horse manure seems to be the favorite so far.]

Here is a link that might be useful: scroll down to winter worm composting

I am not so much into harvesting the compost. I mean I might do that. But I have a HUGE horse manure pile. I am more interested in harvesting the worms. So I don't want them crawling away.
I wouldn't mind just getting some compost out for my own plants.

Oh you lucky people with so much horse manure just lying around. How I envy you!

Oh you would not believe what I have. My number came over and got 2 pickup loads and didn't even touch it.
I got to think I should be able to keep some worms happy in there.

The link below is from the U of CO. scroll down to hypertext "vermicomposting horse manure" for a pretty thorough description of how to vermicompost large amounts of horse manure. Once you get a vermi-herd started in AGED horse manure, and keep it moist, they will multiply like you won't believe! Just keep adding more new manure to edge of windrow and they will follow along, as soon as the manure stops heating up. At some point you will want to make a deal with a local grower or landscaper to sell the vermicompost FOR A NICE PRICE.

Here is a link that might be useful: Vermicomposting horse manure

What you are suggesting sounds like the KISS method.

Here is a link that might be useful: The KISS method