When making spanish cream, how cool should the yolk mixture be before adding the whipped whites?

Well, you don't want to cook the egg whites with the hot custard. Make sure you take the pan off the stove and give it a good stir. Then whip the egg whites. The time that it takes to beat egg whites usually is long enough to let the custard cool. You can test it with your finger. If the custard feels hot, give it another stir and wait a few more minutes. If it feels warm, you should be fine.
Hope this helps. ;o)