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It is still very early, and it appears that you have a shade cloth over them. They prefer full sun. They are quite a bit more heat tolerant than their cousins, broccoli and cauliflower. If you don't see flowers forming in another month or two, then I would think about adding some phosphorus and potassium to the soil.

Generally, in order for them to flower properly, you will want them quite a bit bigger. Patience will be key here.

This article will give you more information on the subject: https://www.gardeningknowhow.com/edible/vegetables/broccoli/growing-romanesco-broccoli.htm

There are a few things to note here. Though they are more heat tolerant as far as Brassicas go... They will have a hard time in the heat of zone 7 (I'm in this zone too). Protecting from afternoon sun can help a little.

Another thing to consider is nutrient content of the soil. Too much nitrogen, or not enough potassium and phosphorus can halt flowering. This may need to be corrected for the future.

You may need to start these indoors under lighting well ahead of spring to give them enough of a mild temperature to finish outside before the heat comes.

This article will help: https://www.gardeningknowhow.com/edible/vegetables/broccoli/growing-romanesco-broccoli.htm

Yes, many of them have the tendency to purple growth. Sometimes it will stay, and many times it will turn the typical light green over time.

This article will help you to give them the best care:

https://www.gardeningknowhow.com/edible/vegetables/broccoli/growing-romanesco-broccoli.htm

Being a direct relative of cauliflower, it will produce in the same manner, generally. Here is an article that will help you to care for the plant:

https://www.gardeningknowhow.com/edible/vegetables/broccoli/growing-romanesco-broccoli.htm

Around now, outdoors, would be perfect! You can start them, indoors, too, but it is about that time to start them outside.

https://www.gardeningknowhow.com/edible/vegetables/broccoli/growing-romanesco-broccoli.htm