Well, you'll need to have at least some dwarves on standby as haulers anyway, to prevent your Legendary Farmer/Brewer/Metalsmith/<insert other useful profession> having to drop what they're doing to move goods to/from the depot, or stacks of xpig tail fibre sockx dropped by the latest wave of invaders, etc.
If you're looking for things to keep them busy here are a few suggestions;
- Set the Workshop Profile of your 'real' Forges to minimum Legendary (or some high level if your metalworkers aren't that high yet) and set up a few extra forges to produce copper bolts, with maximum skill level Novice. Then assign all your Peasants Weaponsmithing. Result: if they have a mood, their highest moodable skill is now Weaponsmithing - so if it's not a possession, you now have a Legendary Weaponsmith for free. (Obviously this only works if they had no prior moodable skills, hence Peasants)
- Branch out into other industries. A working textile industry can account for 10-12 dwarves or more depending on scale (Planters for the crops, Threshers for the thread, Weavers for the cloth, Millers for the dye, Clothesmakers for the final products - I usually double up on these).
- Start experimenting with the military. If you've got that many spare dwarves, one or two lost to a mistake won't be that costly - just make sure they aren't popular...
- Pump gyms are a good way to feed into the military - whilst the stat gains aren't as noticable as through actual military training, when you need to draft replacements, it's easier to identify the less productive members of society from their profession as 'Pump Operator' when all your real pumps are automated.
- Excess stone? Set up about 5-10 masons workshops and set them to make blocks on repeat, and enable masonry on a bunch of idlers - now you have more compact building materials for when they're needed, and have a larger number of skilled masonry workers for any building projects, such as for instance walling off that cavern from the megabeast down there.
Experiment a little - there's many things you can use your dwarves for, and some of them don't even involve them being killed! Have a look through the profession lists and pick something to work on - maybe you'll discover something new and fun for them to do.