Can I use this for multi-warehouse / multi-source inventory?

Yes, but with a tweak. The default model forecasts total demand per SKU. For MSI (multi-source inventory), you have two choices: (A) Forecast total demand, then split by historical source-share. Simple, accurate when source-allocation is stable. (B) Forecast per source-SKU pair directly. More accurate when sources serve different geographies / channels but multiplies your training cost by the source count. For 90% of Magento MSI setups, choice (A) is good enough. Run the per-source split as a simple ratio job after the Prophet forecast lands. Pick (B) only when you have material per-source seasonality differences (e.g., a US warehouse vs an EU warehouse with different BFCM weeks).