it is very heavy and would need to be held in place with something until set

We have used slate on walls in shower areas and for fireplaces. It is installed like any other stone tile using thinset. A normal slate tile is not any heavier than other stone tiles. Slate tile is not as thick as the slate pavers that we are used to seeing outdoors. Natural stone tile may be heavier than porcelain, but the process to install it is not all that different. Ask your local tile supplier for more information regarding installation of slate.