Do we follow the standard (5% of content IIRC) sizing metrics for Crawl DBs when using FAST as our search server, within our Content SSA; or are there separate metrics for FAST (ie. is the crawl controlled with the FAST admin service or the Content SSA.)
Sizing estimates are different
See the Planning and Architecture for FAST document on Microsoft's site
50 GB initial available disk space. As a general guideline, local attached storage is recommended for most deployments unless you have very specific deployment goals. Vital parts of FAST Search Server 2010 for SharePoint services are disk/IO bound, and will have limited access to SAN-specific storage sharing or caching.
Plan for 1 TB disk space per 5 million indexed documents. The actual disk usage is, however, highly dependent of the average size of the items and the selected feature set. Hence, it is recommended to perform a benchmarking for installations exceeding 10 million documents in order to install optimal amount of disk storage
The crawl db scales better for FS4SP compared to the internal search. It is used, but it stores less information so it can hold more than the recommended 25 million items per database. There are no official numbers on how much it can hold yet, but check this thread on the FS4SP forum.
This leads me to think you can probably count 2-3% instead of 5%.