When the table is open, it is legal to shoot a combo of a solid and a stripe. If you pocket the ball you call, you claim the group. You can never hit the 8 Ball first in a combo.
4.9 OPEN TABLE
(Defined) The table is "open" when the choice of groups (stripes or solids) has not yet been determined. When the table is open, it is legal to hit a solid first to make a stripe or vice-versa. Note: The table is always open immediately after the break shot. When the table is open, it is legal to hit any solid or stripe first in the process of pocketing the called stripe or solid. However, when the table is open and the 8-ball is the first ball contacted, it is a foul and no stripe or solid may be scored in favor of the shooter. The shooter loses his turn; the incoming player is awarded cue ball in hand; any balls pocketed remain pocketed; and the incoming player addresses the balls with the table still open. On an open table, all illegally pocketed balls remain pocketed.