Hi Aleks -
Simultaneously querying and co-ranking different collection types is exactly what meta collections are designed for.
Some prior planning is recommended when it comes to aligning metadata mappings, though. You'll generally be wanting to map analogous concepts across collections (e.g. Type =~ Category; Author =~ Contributor). When implementing, either:
- Use shared, consistent metamap.cfg / xml.cfg files (usually defined within the meta collection, and symbolically linked to from the component collections at the file system level), particularly when adding several component collections to a meta collection OR
- Ensure the component collections' xml.cfg files / metamap.cfg files do not have inconsistent combinations of metadata field types (e.g. one with a combination of indexed text, non-indexed text, numeric metadata and geospatial metadata, and another collection with only indexed text)
Component collections may operate perfectly well in isolation, but mixing incompatible index types in a meta collection will produce a Public UI error:  Indexes written in an incompatible format. check_bldinfo()
Overloading metadata fields is certainly feasible: