Yes, it will probably start with mutualism, but to scale to a revolutionary level where the informal market outgrows the formal market you need anarcho capitalism (welcome to reality). Here's why:

1) Dealing in the informal market is often illegal, so for a worker to his own capital goods and only deal with community members is the safest to not get shut down

2) Exchanging goods and services based on the labor theory of value is possible, and probably even preferable among friends / community members. I give you one hour of my time, for one our of your time, even if my time is worth double in the market, because we are friends. (The subjective value of the good + the value of helping a friend makes the exchange still worth it, even if the product alone isn't worth it)

So yes, mutualism probably works best on a small scale and under a strong state.

However, if the state weakens and the paralell economy wants to compete with the formal economy, mutualism fails:

1) Operating on a bigger scale becomes safer because enforcement is weaker, so division of labor becomes an important factor to compete. The capitalist providing the right capital goods is an important part of it.

2)To scale you have to deal with strangers, so you HAVE TO exchange based on subjective value instead of value based on labor. The added value of helping a friend isn't there, so the deal will suck for (at least) one of the two parties compared to a deal based on subjective value, simple because you use an improper price system. This will prevent the exchange from happening, and it will move back to the formal market.