Is it possible to change rate of VAT on checkout page? For example I want to add condition if user is guest and commercial then use 20% rate else 0%
That is currently not possible, but there is a feature currently being implemented which allows for custom tax calculations.
Search for "Interface for tax services" in the public roadmap.
That feature is designed to allow to connect to external tax providers for more complex tax calculation e.g. the US, but it should also allow to change the taxes by rule etc.
However keep in mind that in that case the prices in the store prior to the checkout page may not be accurate, this can have some legal implications depending on the country where the shop is operating.