@ Alon - I found information from this site to be insightful (https://garden.org/learn/articles/view/362/). One area of this article is for smaller plants. "Not all tomato plants need staking. Determinate tomatoes stop growing at a certain height -- usually when they're fairly short. They stop growing because the main stem forms a flower bud at the top that produces fruit. Most of the determinate varieties are early types, and they're bushy plants with short, stout stems that support them pretty well."
@ Amanda - If you're going to damage the rockwool then cutting some of the plants base would be a better option. As they get bigger the space will become limited. In our experience, no more than 3-4 full grown tomato plants would fit in the Niwa system. How many of your seeds germinated?
@ Amanda - Thank you for picture and how exciting to see them grow! We suggest to support your plants with a small supporting stake. You can put a twisty tie from the stem to the stake for support as they grow. See below a link that can help you. Moreover, 10 plants is a lot for the Niwa box. We suggest transplanting them when they become large enough when light isn't accessing all your plants properly. Please send us a photo in about 10 days to see what's happening. Lastly, can you please send us a close up of the vermiculite to make sure nothing bad is happening.
Helpful Link >>> (https://www.thespruce.com/how-to-stake-up-houseplants-1902694)