Some attachments will work at the lower GPM's (although slower). Others will not. Many skidsteer attachments only require around 10 gpm. Some of the bigger ones are closer to 20.
A tree auger designed for 20 gpm will probably be fine at 10 gpm at the same pressure, but just be half as fast.
Something like a mower a rotary broom might not get spinning fast enough to do its job well, however.
A trencher might be marginal.
A tiller owould probably be OK, but slower.
A jack hammer - not sure how they work, so I don't know. Might just be half as many blows per minute.
As another poster pointed out, your tractor hydraulics might not have enough cooling to run sustained full power loads like a hydraulic trencher or a tiller would demand.
For my 4100, I built an adapter to the mini-skid-steer attach plate used on the Toro Dingo. The attachments are scaled-down versions of the skid steer stuff, but are available to rent and cost half as much per day as well.
When not using it, I take the adapter off and use the normal bucket attach for my bucket and forks.
- Rick