I don't have a lot of experience with serranos, but for jalas, they're fine to use as soon as they're of a decent size. If you press on them and the flesh yields, the walls are too thin; otherwise they'll be fine. It depends on the variety, though: I've had some that are smaller and ripe, some that are bigger.
Red yields a different flavor, and can be great roasted on the grill, makes a very different texture and flavor for salsas versus the sharper green ones.
Depending on heat and water, jalas will develop cracks and lines running the length of the fruit. They're not beautiful, bu they taste the same.
What is patriotism, but the love of good things we ate in our childhood?
-- Lin Yutang