Any flow & sizing option can be applied globally, or to individual sub menus. The example menu below has the 'Main Item 2' sub menu set horizontal.
Main Item 1
Title Item 1
Sub Item 1
Sub Item 2
Sub Item 3
Main Item 2
Sub Item 1
Sub Item 2
Title Item 1
Sub Sub Item1
Sub Sub Item2
Sub Sub Item3
Sub Item 3
Sub Item 4
Main Item 3
Title Item 1
Sub Item 1
Sub Item 2
Sub Item 3
New Title
Sub Item 4
Sub Item 5