(1) You can replace the long stem cauliflower with a regular white cauliflower. Regular white cauliflower is softer than long stem. Shorten cooking time by blanching it for 1 minute instead of 2 in step 1. During stir-frying, mix sauce in a gentle manner, to avoid break the florets.
(2) The amount of sugar you should add depends on the flavor of the tomato. If you don't like a sweet dish, reduce the amount to 1 teaspoon. You can always adjust flavor by adding more sugar in the end. The dish tastes best when sugar is well balanced with the sour flavor of the tomato, but doesn't overwhelm it.