Nitpick, but there are certainly parasitic fungi which do not have a symbiotic relationship with plants. Also only some fungi have a mycorrhizal relationship with plants. Most are saprophytic meaning they decompose dead organic material.

This is of course extremely beneficial and required for a closed ecosystem, but the symbiotic relationship isn't as direct as mycorrhizal fungi and is really part of a much more complex network of soil organisms that all need to be at play. Simply introducing a saprophytic fungi to your soil will probably not result in any benefits to your indoor cannabis garden.