Hi Hayleigh,
> ... Isn't this widget supposed to support up to 1000 features?
I believe the limit for the Add Data widget is 1000 features or if the file is more than 1 MB in size.
> ... which is a very generic error, assuming if it was over the limit we would get an error saying the file is too big?
Please contact esri Tech Support and log an enhancement request; I believe the generic error message is expected behavior.
Hope this helps,