We advise pool owners to allow salt to mix in for a day or two before turning on the SWG, as wide swings in salinity are hard on an SWG. I expect that advice would apply to your Mini, too, though in a spa the amount of time for a good mixing would be substantially less than for a pool. Just be a little patient before first "flipping the switch," and give your salt a chance to mix in well.
Chlorine leaves salt behind as it gets used up. Depending on how old your spa water is, and how much chlorine you've added to it, and your fill water, you might already have quite a bit of salt in the water. Don't just add the amount of salt as per Pool Math as if your starting point is 0 salt. You need to test the existing water first, and plug that into Pool Math so it can calculate the correct amount to add. Otherwise you could end up with too much salt. No biggie for a spa, I suppose, as you can just dump some water, but a proper salt test kit and Pool Math can help you dial in just the right amount.