You can use sneak in combat. Base AP cost is 6, minus 1 per point in sneak. It's actually an incredible damage buff if you get to 5 sneak with Guerilla (doubles damage from an attack made while sneaking).
You can switch, and you CAN use both, but as an example, if I want to be a great (damage dealing) dagger user and archer, I need:
Talents
Backstab - dagger damage without this is next to nothing
Quickdraw (requires 5 marksman) - reduces AP cost of bow from 4 to 3
Attributes
Speed, high enough movement (can also be boosted with equipment) to position myself behind the enemy without too many APs used
Perception, high enough to hit enemies that aren't at very close range
If you specialise in only one of these areas. It's a far more difficult thing to balance the attirbutes than the talents, and you could just trade excess talent points for attribute points later in the game if you want.
That said as I alluded to above, you don't have to be the one dealing damage in the party. Another perfectly viable strategy with an archer would be to not join combat until after the rest of your party (unlink), join while already in stealth, then use haste and save AP your first turn, on the second turn fire a few arrows/special arrows and use Walk in Shadows. This way there is no threat to you assuming you're behind the enemy. Later when Sneak is high you can use that instead of Walk in Shadows.
The game is incredible in the amount of choice it gives you in how to build a viable party, but I think a lot of people just end up focusing on a per character basis.
