If I have harissa, but not in paste form, how would I adjust the measurement to approximate the same amount of flavor (without affecting texture)?

The texture won't be affected. Harissa paste, no matter how potent, will on;y be used ib a matter of teaspoons to tablespoons here. Never enough to affect your texture.

As it's a soup it shouldn't matter much. Just taste it until you think it's right. Harissa is available in tube form http://search.culinarydistrict.com/search?view=grid&w=harissa&x=35&y=5 from Surfas. It's cheap but you might want to shop for some other stuff to offset the shipping costs.