To be an SI you need an inome of 250k+ or 2.5m in assets. This needs to be certified by an accountant and is valid for two years.
The main perk is that you are legally allowed to be offered securities without the need for disclosure documents. So a company can raise funds from you alot quicker.
So yeah, as EB said, being a Top20 holder has nothing to do with it, although there are definitely SIs among MNMs Top20.