QIf there are 3 cards on my field and I play this spell, can I select which 2 followers to summon?

AYes. Whenever a spell or ability would summon more cards than the field's card limit allows, the spell or ability's player may select which cards to summon.

QIf my field is 1 card away from its card limit, can I choose whether to summon a Viking, Steelclad Knight, or Knight with this spell's ability?

AYes, you can.