[SIPC instead of FDIC insurance - coverage is about the same]
You left out one detail. Technically if you have $100,000 in Money Fund A it means you hold 100,000 shares that are priced at $1 each. SIPC will protect you from losing your 100,000 shares but cannot protect you if the value of each share falls below $1. This is unlikely to happen, but is legally possible. There were some scares about this happening in 2008.
If you want to be rock solid safe you could choose a money market fund that is backed 100% by US treasuries, but you would earn less interest.