It looks like you didn't plant it deep enough. If you see root exposed still like it looks like in that picture, take some mix from the rim of the container and rake it in with your hand gently to form a little mound around the base of the stem.
You mentioned hydro but what is your medium exactly? A soil-less mix? Sphagnum with perlite?
It really doesn't look like over wet to me; surface looks quite dry except for where water was sprinkled on. Fungus gnats are little black flies that hover above the media surface and they primarily eat dead vegetative matter or microbes in the mix. A few is not really a concern; get some yellow sticky traps (hanging or sticky stakes).
I don't really understand watering x volume of water every x days, because for one thing grows vary in container size, media moisture retention, lighting, temperature, air flow, humidity, et al. and watering needs can vary with plant size and changing environmental conditions.
It is okay for a media to retain moisture, plants need moisture. A good media also holds air when wet. The thing is that the primary materials used in potting mix and one that you mentioned: sphagnum peat moss, as well as coco coir, will actually become hydrophobic when they dry out. They repel water initially. If you've ever tried to hydrate plain dry sphagnum peat (as typically sold in bales) you will witness this first hand.
Easiest way to tell if you need to water is to lift your container, and if it is light it is dry (heavy means wet). Combine that with sticking your finger in the mix nearer the rim. If it is dry, water thoroughly. This is easier if you don't start in a container too large. But, remember what I said before: if it is really dry it wont even want to absorb water. You'll need to start by watering slowly, preferably sprinkling the soil surface with rain-like consistency. The other thing is, and especially if you started with raw peat (which should also be limed) use a wetting agent such as yucca extract, or perhaps fragrance free castile soap. Using a wetting agent will make make the media (sphagnum or coir) easier to water, and help distribute the moisture throughout.
You don't want dry spots as that will affect root growth. Your media should behave more like a sponge, with moisture evenly distributed; and there should always be a little bit of moisture. Over watering occurs due to lack of oxygen in the rhizosphere, because water itself doesn't dissolve oxygen very well (and less in warmer water). A good media will hold air when wet, however, and OW wont happen so long as the media is allowed to dry out more or less between each watering. Also don't let pots sit in stagnant water. It should go something like wet --> slightly moist --> wet -->slightly moist. After you water if there is run-off dump it after 20 minutes. You can also water from the bottom, but again if anything is left after long you need to dump it (and use a wetting agent still).