Sounds like slightly too much juice. Not dangerous. You can blow the extra juice out: Wipe down the threaded end and blow on it onto a paper towel (after removing cartridge). Or you can just vape it out.
There is a three month warranty on the Leo atty, so if you do manage to blow one soon it can be replaced. At least I assume user error is a valid returnable issue

If you are using PG juice on an empty cart you can get more then 18 drops. More like 25-30. There is no need to stand them up for any time, and this may be overjuicing your atomizer. I just got a third Leo and barely primed the thing before putting the cart on and vaping.
It sounds like your juice is all PG but if you purchase juice that is more then 10 or 20% VG you might have wicking issues. If you look at the tips thread on this forum you will find the highping mod which works pretty well (for me). To explain it simply: take close to half the cart filler and roll it up. Fill the cart about 2/3 full of juice and put the rolled up bit on top with the edge of the roll facing up. drop a bit more on top and you are ready to go. Many people replace the filler with other materials... look around ECF and you will find hundreds of pages about them. Search for fluval or BFD Mod or Blue Foam...
You shouldn't need more hardware for quite some time with two royal kits (though you might want some). Juice should be your main expense for quite some time.
The Leo was my first e-cig as well, and I love it. It takes a little while to learn all the little tricks to get everything working the way you want it to with VG, but it got me off cigs in a day. I hope you have the same luck!