The trouble with ‘Slippery Slope’ and ‘No True Scotsman’ is that they themselves are not fallacies. Invoking them without proper justification is the fallacy. The same sort of thing happens all the time with ‘Appeal to Authority’, you can probably trust a scientific consensus about a subject in which they are all experts, but you probably shouldn’t trust an individual expert on a topic for which they are not recognized as an expert.
For an example of Slippery Slope: Fascists will absolutely try to demonize the most available target, and then because they always need an out-group, they continue cutting at what they consider the ‘degenerates’ of society until they are all that remain. (And then they find some new definition of degenerate)
“No True Scotsman” is valid in that there is at some point by definition after which you are no longer talking about something. “No true vegetarian eats meat” is valid, as this is definitional. “No true member of Vegetarians United eats meat” lacks proper justification, and refers to an organization, not a proper definition. This gets really messy when people conflate what group people are in with what they ‘are’ or what makes them a good example of a group. Especially when religion is involved.