Why, when the Power of Ten is used on Simon and Snarf, do they not go away for good? Because, as revealed later, Zirconia and Spinell's powers had been drained during their imprisonment, so the spell wouldn't be at full strength.
Sapphire is in an unusually mischievous mood in episode 25, casting a spell on Topaz for no real reason. While seemingly random, in the final episode the girls have all lost their tempers and cast spells on one another; this could be an early sign of what's to come.
That, or it's leftover influence from the previous episode/arc.
The Ancients were around the same age as the girls when they fought Simon 3,000 years ago. That means Zirconia and Spinell were either married as teens, or they were married after being transformed.