Gender difference for Vieja are sometimes hard to see even when mature.
If yours are young there is probably no way to tell with simple photos.
Depends on how territorially entrenched the GTs are, whether or not they will tolerate another cichlid. Time could help, especially if the Vieja stays out of their way.
But Vieja get large, and can be tough, so if the Vieja lasts, it may turn the tide on the GTs , because it will get large and tough enough to assassinate them both.
That's what I'm worried about. Will there be a warning sign for the Vieja or will I just wake up to a dead fish or two that the viejas left behind?