You can add all the options of <cmd>setupcolumns</cmd> to <cmd>startcolumn</cmd>.
Normally, columns are automatically balanced with the text distributed over the number or requested columns. You should be able to enforce can provoke a new column with switch using <cmd>column</cmd>, but that's broken (at least since 2006). <texcode>\startcolumns[n=2]\input knuth\column\input tufte\stopcolumns</texcode> <context >\setupbodyfont[9pt]\startcolumns[n=2]\input knuth\column\input tufte\stopcolumns</context>
Column balancing fails in "lines" mode (e.g. for poetry).