I open Edit > Schema to change the fields to show less decimals but I'm not allowed to change the 'Type:' which has 'float32' greyed out.
Correct. Like most high end DBMS, you don't change types in a schema in Manifold. Instead, create a new field that is the type you want and then use Transform to copy your data into that new field. Most transformations between types for numbers happen automatically, but if need be you could use CAST for the transformation (see the examples in the Transform template topics on using CAST).
What would be nicer would be to have Style enabled for table windows, so you could just force the display format into being whatever you want regardless of the type. I, for example, don't like having a bunch of "0" digits in my date time fields when all I care about is the date, not time.