The api phosphate test is pretty inaccurate, but a .5 reading can cause algae issues if it is correct.
In terms of a skimmer on a 18 gallon tank, you can get away without one. Due to small tanks being so small, it is easy to export the nutrients with water changes rather than running nano skimmers, which can sometimes just be ineffective due to the size as well.
In terms of having a heavy bioload though, you are correct. I would also be wary of those 3 fish having enough room to swim around...especially the firefish. This could make the skimmer effective on your system, but long term I wouldn't expect a firefish to do well in such a small space along with 2 other fish that I would want to house in a 30g and up.