I have a moss ball and java moss in my 20L and find that's alot of moss. Unless you really want a moss ball, you can probably get away with just the java moss. It'll grow well enough it'll start needing to be thinned out regularly. A floating plant is a good idea and they're good for removing excess nutrients, which will help keep your algae population in check.
I wouldn't buy a new light; the one you have is fine. More lighting nerdery below:
The importantthing to know is that plants uselight in the 450-700nm wavelength. Kelvin is largely for human benefit, but I think there's some correlation between it and depth of the tank. If you stay between 5K and 10K you should be okay (though I would be *REALLY* surprised if you could find a 10K bulb for a 5g tank). The lower the Kelvin, the more red the light will look and subsequently, the more it will affect the colors you perceive in your tank.
I would do a water test before you put it in. The reason pool filter sand is recommended is because:
- It's larger grained and less prone to stay suspended in your water column
- It's generally (always? I'm not sure about this) made of silica sand, which is chemically inert. Play sand seems to have an equal chance of being harvested from somewhere local and mineral composition of it can vary by region.
- This can complicate your water chemistry, though I don't know how much or whether it matters. (I'm an engineer by profession, so I tend to stick pretty closely to numbers)
It doesn't necessarily need to be a toothpick. You just want to basically jab the substrate to collapse any bubbles that are formed from anaerobic decomposition before they burp and send soil spewing into your water column.
With plants, you should be able to support a higher bioload, but I've done zero research on it. I generally just ignore snails as part of the bioload, though I just recently read that they can be massive ammonia producers when dead.