As an example, below are the files related to two movie files — which should I transfer to my 5TB external drive, and which am I safe in deleting? (to help distinguish, let's use the name "Q" for icons with that letter and "Grey" for those with black and grey stripes)
Based on the information given, it is impossible to answer your question in a simple manner.
Generally speaking, a "generic icon" (one with the letter "Q" on it) merely indicates that the data in the file is not "natively" compatible with QTX or does not contain "media" data while those having a picture icon are "normally" QTX/QL compatible. It does not indicate the quality of the content, its source or intended use by you—all of which may be factors in deciding which files to keep and which to delete.
On the other hand, It does appear that most of your "generic icon" files are extremely small in size which could indicate they are reference, single-frame, corrupted or incompatible low-quality versions of your other files having the same filename. (I.e., you may want to ensure the files are playable or required for the playback of other media files before getting rid of them.)
As to larger files having the same filename and same storage size, you may have to compare the individual copies to ensure they play the same before deleting those you determine to be duplicate copies. I.e., MOV files can be edited (e.g. trimmed) and the new start and/or end playback points stored internally within the file so that they externally appear to be the same size but in actuality play differently in QT-based media players than they do in non-QT media players. In short, the only way to really be sure which of these files to keep and which to delete is to physically go through the files and check them individually.