Online platforms like YouTube and Instagram have enabled the platformization and monetization of creative work, allowing content creators to derive revenue and thrive in a creator economy. While much work has been done to understand content creation on single platforms, the creative practice often involves content creators' agency and practice to interact with multiple platforms and make strategic decisions to optimize such interactions. In this paper, we use an interview study with 21 cross-platform creators to understand how they negotiate with platforms in their creative practices through the construction of creator ecology. We found that participants developed priorities among platforms based on varied criteria, paid attention to cross-platform content synchronization, and stressed managing and converting audiences across platforms to grow their fanbase. Our findings highlight the complex interplay between creator agency and labor, as well as yield implications for future design possibilities of creator empowerment and support.