In general Road cassettes go up to 27T, so have mechs that track that range. These mechs will usually run up to 30T but Shimano do not guarantee it.
MTB cassettes run up to 36T so have a mech that will track this.
Saint is an adjustable mech for road or MTB cassettes.
The cage length (short middle or long) is merely a means of taking up the chain slack and is usually approx 25, 35 and 45 teeth range measured as the total difference between large+large and small+small sprockets+chainrings.
So in summary, get a mech to match the cassette max sprocket with a cage to match the range of front and back rings.
PaulD