Is there a way to import image titles or captions from a text file?
There is no automated method for extracting image titles or captions from a text file. (Neither JuiceboxBuilder nor any of the plugins have such functionality.)
With JuiceboxBuilder, you can automatically use the image file name or IPTC data (or enter text manually). (If you use Adobe Photoshop to enter your IPTC data, then use the IPTC Document Title field for the image title and the IPTC Description field for the image caption.)
It would be possible to build a gallery's XML file, extracting image titles or captions from a text file, using a script of some kind but this there is no such automated solution available currently. Also, the solution may may be unique to your own scenario. It is perhaps unlikely that other users would have text (that they would want to use for image titles and captions) stored in a file using the same format as yours.
Can an image gallery handle more than 1200 images?
Yes. Juicebox-Pro features an imagePreloading configuration option (which can be set to PAGE, NEXT, ALL or NONE) to determine how many images are preloaded by the user's browser (to prevent the browser from trying to cache all gallery images as soon as the gallery loads).