This got me wondering about the validity of this 'rule', so I did a little checking. Some time ago I had downloaded the SOWPODS word list and created a database from it, so time to do a few searches.
The results were illuminating:
- 475 words containing 'ei'.
- Only 49 of those words contained 'cei'.
- Of the remaining 376 words, 109 have the long 'a' sound.