You can safely use .44 special loads in .44 mag brass, but I'd recommend you do so with cast bullets. It's a lot easier to lodge a jacketed bullet in the barrel with low velocity loads than it is a cast bullet.
In this specific case I also wouldn't use a starting load for .44 special--at the risk of alarming lots of folks here I'd actually recommend starting at the max load in the .44 special data and working backwards until you find the accuracy and recoil to your liking. This is normally exactly what you wouldn't do in load development, but in this case you're using .44 mag brass in a .44 mag handgun, and even max .44 special loads in the longer magnum brass are going to generate less pressure than the starting loads for .44 mag data.
When loading light charges in magnum cases, also be really careful you don't double-charge any cases. Unique is pretty bulky compared to others, but it's still something to keep firmly in mind.
Mike