Thanks for asking
The conventional wisdom is, "If you can't find a loading manual printed by a reputable publisher that lists a bullet/powder combination, don't load it."
What you are contemplating is developing a load from scratch. Something that conventional wisdom says should be done by a ballistics laboratory (where they have instruments for measuring pressure and velocity and they don't mind occasionally blowing up a firearm and also have blast shields and other safety measures).
The typical handloader should not do it.
That's the bad news.
The good news:
Unique is a fairly fast powder and is forgiving of light loads (where slow-burning powders are definitely NOT).
A handloader paying attention to all the various factors can push the performance envelope (with caution) if taking proper precautions (basically being your own lab). To my mind, doing what you contemplate, your greatest risk is getting a bullet that does not leave the barrel. The next bullet, encountering a barrel obstruction, may do damage to your gun, to you and to bystanders.
To mitigate that potential would involve at the minimum, a chronograph. If you get inconsistent velocity or velocity below 500 fps, I would suspect that some time or other you will have a bullet fail to leave the barrel. Also, you should keep an eye on the brass. If it has soot on the outsides, it is likely because there is not enough pressure inside the case to cause the case to expand enough to seal against the chamber walls.
Those are only two of the things one should be aware of when developing new loads in unharted (unpublished) territory.
Here's my advice. Email or call the makers of Unique and ask them their advice.
It seems to me that this particular quest (low velocity lead bullets using a fast powder) is not particularly dangerous, as these things go. But I definitely would not try it without a chronograph.
As a side note: When I taught my friend to reload for his 500 S&W, I suggested Trail Boss powder. It occupies a lot of volume for the energy released. Difficult to make a double charge or a missed charge with Trail Boss. We loaded a bunch of 350 grain slugs that clocked 700 fps. So now, one of his favorite loads for the 500 is a 1,000 fps (thinking 700-800 is too low a pressure for reliability). Lesson: Get a chronograph.