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18"-24" is how far apart to plant them. They get to be about 1' tall.

In your area it is best to keep the plant potted and treat it as a houseplant (bright, indirect light), allowing it to spend summers outdoors (in a shaded area) and bringing it inside during fall as temps begin to drop.

While most repotting is performed in spring, it is ok to go ahead and do this if yours has gotten out of hand. Move it up another pot size or two and use a well-draining potting soil. Also, make sure your new container provides adequate drainage to prevent root rot. This article will help with repotting: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm

The sap is the only poisonous part and since there's no worries in the plant being eaten, your only concern would be contact dermatis (itchy or burning rash) if you are sensitive to the sap. Wearing gloves can provide adequate protection during the potting process to avoid this, though unless you break a leaf or stem off, your're unlikely to encounter this sap.

This is the sap from the plant, which is toxic if ingested and will cause contact dermatitis of the skin (reddening, itching, stinging) in sensitive people. Wash your hands thoroughly and it should eventually go away on its own. If not, seek professional help. In the future, you may want to wear gloves when handling these plants.

Rhoeo needs well-drained, dry soil and can develop rot rot in wet soil, so make sure you can provide an appropriate site for it. A good companion would be any plant that also enjoys low moisture conditions. See this article:

https://www.gardeningknowhow.com/ornamental/flowers/rhoeo/growing-rhoeo-plants-in-the-garden.htm

The most important aspect of preparing the bed for Rhoeo is to improve the soil drainage if necessary:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-well-drained-garden-soil.htm
http://goodlifegarden.ucdavis.edu/blog/2011/08/how-to-improve-your-soil-drainage/

You absolutely can! Germinate these like you would any garden vegetable. Here is an article on the care of these: https://www.gardeningknowhow.com/ornamental/flowers/rhoeo/growing-rhoeo-plants-in-the-garden.htm

There are two main causes for this.

One will be lower temperatures. Anything under 45 F will start to damage the plant.

The more likely situation is overwatering. They are very drought tolerant, but not tolerant of soils that do not dry out thoroughly between waterings. Overwatering leads to fungal and bacterial infections, and sometimes these can be very difficult to treat.

I would recommend a fungicide, then to let the plant's soil dry out, completely, down to about 3 or 4 inches between watering. Cut off any growth that dies back more than halfway.

These articles will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

https://www.gardeningknowhow.com/ornamental/flowers/rhoeo/growing-rhoeo-plants-in-the-garden.htm