You asked about skid steer loaders rather than the toolcat, so I will address the skid steer. If you are going up a steep hill, then you should be traveling in reverse with a skid steer. There is a potential to tip over backwards when traveling forward with skid steer, even with bucket full. Keep the bucket low even traveling backwards with a skid steer. It could tip but the bucket will stop it from tipping further. I don't have any maximum slope degree for a skid steer when traveling backwards. If it is too steep, you won't have the traction.
If you have to travel forward uphill, then I don't have any magic degree for slope. It will depend on how heavy the load is in the bucket and how much risk you can take. Can you back up the ramp and then turnaround?