A.: It depends. Slack is the default place and that content messages should only be written in Trello once there is a clear set of project actions to be shuffled and prioritized - anything that involves discussion in order to define project and their boundaries is better off in Slack (maybe in its own channel). So, in general it is about task orientation. If it is related to an upcoming or ongoing task, then Trello. If not, Slack. In Slack, you might want to seek out a channel where people gather around relevant interests (rather than posting everything in the general channel). Also, there are some neat integrations between Trello and Slack that you could make use of! When using these the question might not even arise.
A.: More than 20 lines go into a document might be a guide here. Also consider the options that exist with, for example, a Google Doc; people can comment on specific sections, edit a document with you, etc. On a messaging platform you will generally just go back and forth in a conversation where the starting point moves into the distant past (rather than evolves as in co-editing). If you create a document, make sure it is accessible and that you share it with the right people. And if it is related to a specific task, consider Trello!!!
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