Does GISquirrel support multi-user editing?
Yes, and you only need an ArcView / Basic ArcGIS licence.
How does GISquirrel resolve conflicts if two users edit the same feature?
It prevents this situation arising by locking features as soon as they are being edited by any user.
Can I load data from shapefiles and personal geodatabases?
Yes, in fact you can import data from any vector layer that you can add to your map.
What editing tools I can use?
You can use all the usual ArcMap editing tools.
How do I append features to an existing GISquirrel feature class? The Import Layer tool does not allow me to target an existing table?
You can start a GISquirrel editing session on the feature class, then select all features from the new layer and paste them into the editing layer, then save. This passes the features through the same import and cleaning process as the Layer Import tool. If the incoming layer is very large, this may have to be done in batches.