I've seen and heard that having iso as low as 100 can be negative to the picture. I wondered is this true? and if so, why?

This can be the case with cameras where the "true" base iso setting is 200, so that iso 100 is done with in-camera processing, rather than adjusting amplification on the sensor itself. The sensor cannot physically go lower than iso 200, so the image at iso 200 is simply taken at +1EV overexposure, then processed to bring the brightness of the image down -1EV to simulate ISO 100.

However, this reduces the dynamic range of the image. And it's why using an "extended" ISO setting at either the high or the low end of the ISO range is something you only want to do if the tradeoff is worth it. Using a very low ISO can be similar to having a -1EV neutral density (ND) filter built-in, and could still be worth it to get a slower shutter speed, or to use a bigger aperture than you could if you were limited to ISO 200.

At the higher end of the ISO range, the reverse happens. If, say your camera is actually limited to ISO 3200, but you have "extended" settings for 6400 and 12800, those settings are achieved by the camera using ISO 3200, and underexposing, then "pushing" the exposure brighter in post-processing. This will probably not only enhance the noise than exists, it will also reduce dynamic range, just at at the low end.

There are a lot of current model cameras that have "ISO-invariant" sensors where theoretically shooting at a lower ISO and boosting it should not create additional noise. But most of these sensors may only be invariant over part of the range, and may not be perfectly invariant. And, regardless, if you use an extended ISO setting at either end, you're reducing dynamic range.

If the picture is perfectly bright enough then why should iso need to be high?

If you are not using extended ISO settings, but are within the native range of the sensor amplification settings, there isn't much reason, unless you need it to get good exposure with a faster shutter speed or smaller aperture setting.

Additionally, do other settings also affect grain, such as aperture and shutter speed, or is it purely iso?

Actually, ISO has nothing to do with "grain." Grain is a film term that related to how on a film stock, the ISO was determined by the size of the silver crystal grains on the film emulsion. The faster the film was, the bigger the grains are, and the grainer the images will be.

But with digital, we don't have any silver crystal grains or grain size. What we have is noise. Noise can come from the photons (not) hitting the sensor (a smaller amount means lower signal and possibly higher noise; i.e., darker areas tend to be noisier); from all sort of sensor electronic issues, like "dark current" heat, residual signal from the sensor not being completely cleared, or analog amplification.

But in general, to get grain with a digital image, you have to post-process it in as an effect. Grain is not uniform in size and far more random than noise typically will be in an image.

Basic beginner instructional material will often be wrong and confusing by calling digital ISO "grain" or "sensitivity". But those are how ISO worked with film. With digital, we have "noise" and (mostly) "amplification" instead. Some of the amplification is analog and adds noise, some of it is digital, which may or may not. So just increasing the ISO may not actually add more noise, depending on which methods are used under what conditions. It's not always as simple as lowest ISO => least noise; highest ISO => most noise.

The other exposure triangle settings typically do not add noise to an image, unless they require you to increase the ISO to get a good exposure.

In a nutshell is it bad to have a very low iso, almost the lowest the camera can do, if the picture is bright enough?

Depends on whether you're using an extended ISO setting.