From what I've been told by others under 20g you won't need a protein skimmer as long as your consistent with water changes. I have an 8g and it's doing well, just make sure you're using rodi.
I rarely test if I do I have someone check my phosphates. Water change every 7-8 days. The only time I had a mini cycle is when I first started and overstocked (I also didnt use LR but I can attribute the mini cyce to overstocking and excessive feeding).
I'm sure others here can make recommendations on test kits.