Hi,
Higher stocking by itself is not an issue, however, higher stocking rates produce higher levels of organic waste, which if left unattended can trigger algal blooms in a high light tank. As with non-planted tanks you can add as many fish as you want as long as you keep the tank clean.
Higher stocking rates also produce more competition for Oxygen during the night as the plants consume Oxygen while not producing any. Oxygen levels can be depleted by early morning putting the fish at risk. The low oxygen levels combined with high residual CO2 levels can be a problem. Of course there are ways around this such as supplementing with an airstone during dark periods and so forth. The tutorial gives general guidelines but you are free to break the rules-of-thumb as long as you understand the consequences and are prepared to resolve the issues that arise as a result of deviation.
Cheers,