Predelay is just a simple delay of the signal going in to the reverb. So normally I would not use a separate delay for the purpose of predelay.
However, you might want to use a separate delay if you wanted to impart some character into the reverb. For example, you might use a tape delay before a plate.
If you're using a separate delay as a predelay, set the delay to 100% wet. But you can also experiment using a delay mixed into the straight signal going in to the reverb. This will not give you predelay however, so you may want to add predelay using the reverb's built-in predelay.