3-6 mm flies. The adults are harmless but the young maggots live in the soil and feed on organic matter. They thrive in moist soil and affected plants will be stunted and foliage may fall off. The best control is to remove plants from the soil, wash the roots and repot in sterilized soil. I wouldn't worry about these guys too much, for in all my years dealing with plants I have yet to see damage caused by fungus gnat maggots.
Mealy bugs are tiny, white insects that look like tufts of lint.
They are another of the sap suckers' and bad infestations will cause stunted growth and eventually kill the plant. You will find them on the underside of leaves and where the leaves branch off from the main stem of the plant. Use the same controls as for aphids. Mites are generally too small to be seen without a magnifying glass. They are brownish in colour and look like dust.