How to select all units of specific types from a group in Starcraft 2
Solution 1:
You can ctrl+click the unit in the hud. That will select the same type of unit in the group even if they are not visible on screen.
Solution 2:
There is a certain key combination to cycle through subgroups in a group. Something with TAB.
Alternatively you could double-click on one unit which selects all units of the same type.