Is Koicha a Matcha espresso?
Yes, somewhat similar in terms of powder to water ratio. It uses more matcha with less hot water and is simply a strong Matcha cup. But Koicha is not casually consumed like expresso. It is a part of Chaji, the official (or long) version of tea ceremony which includes food and dessert servings.
I have never seen a cup of Koicha. As mentioned, Koicha can be only seen at an official tea ceremony and we seldom drink Koicha at home unless you really like a strong matcha cup. Most of us are more familiar with a cup of Usucha (with the foam). Matcha Usucha is still a ceremonial grade matcha with very good quality, umami and sweetness.
Do I have to use Koicha only for Koicha? You can use Koicha for a Usucha style cup that is even more enjoyable, but not vice versa because simply it would be bitter if you made a strong Koicha cup with Usucha matcha.