Artificial Intelligence (AI) adoption in micro-sized firms is an important theme in small business and digital transformation research. Generative Artificial Intelligence (GenAI) is changing workflows across the creative industry and creating new opportunities for small firms. While large organizations often manage AI adoption through formal policies and structured processes, micro-sized creative firms usually operate with flexible structures and informal rules. As a result, there is limited understanding of how AI adoption is managed in organizations with flat hierarchies and informal work environments. This study explores how trust functions as an informal governance mechanism for AI adoption in micro-sized marketing firms with flat hierarchical structures. The study was based on the Socio-Technical Systems Theory and Emergent Strategy Formation, and an abductive, qualitative multiple-case study methodology was followed. Empirical data were collected through seven semi-structured interviews across five micro-sized marketing firms in Sweden and analyzed using Template Analysis. The findings show that trust operates as a multi-dimensional governance mechanism, stabilizing AI adoption across vertical, horizontal, individual, and external relationships. Rather than relying on formal rules, trust enables decentralized experimentation and functions as a coordination system that substitutes for formal technological governance. Furthermore, trust facilitates the joint optimization of social and technical subsystems, allowing bottom-up practices to evolve into an emergent organizational strategy. Organisations can use these findings to manage AI adoption in flat, low-hierarchy environments by relying on trust-based coordination, so employees can experiment freely while still working towards a shared strategy.