Should I follow the same rules when using a flash (direct/bounce mode)?
It depends on how you plan to use the flash, and more specifically, how you plan to balance the flash against the ambient.
If you are planning on balancing the flash against the ambient so that the flash acts as fill (i.e., most of the light for the exposure will come from the ambient, and the flash is only used a little bit to "fill in" the shadows), then using similar exposure settings to what you'd use for ambient is a good rule.
However. If you plan on using the flash as your main source of illumination, then your ISO setting plays a role both in where you set your ambient exposure, as well as in trading off against how much light your flash can output.
Shooting at iso 100 or f/8 indoors can be asking for more light output from a hotshoe flash than it can generate. So you may have to increase either setting to a comfortable medium just to get an adjustable range of light from your speedlight for the image. But using a lower setting can help you underexpose or kill the ambient light in the image the way you want.
My starting point with flash is to typically set mid-range values: ISO 400, f/4, and my camera's sync speed. And then adjust from there, depending on what I want for my lighting ratios, keeping in mind my flash is only powered by AAs and can only output so much light.
See: Strobist's "Lighting 102" assignment on Balance.