Ok, so in the HTC Mail app, to select all, first select one (check one of the check marks next to the message), then hit menu and there is a Select all option.
In the gallery, you first chose the action before selecting the pics and it will then let you chose multiple ones. For example, to delete multiple pics, go to gallery, and while looking at all of the pics in a folder, select delete, then check all of the ones you want. If you want to delete all, again, just select one, then hit menu for a "delete all" option.
Hope this helps you guys!