You should be just fine, but honestly with a house built in 1921 I'd have an electrician come in for a quick consult about your plans.
There are absolutely no issues I am aware of around having geckos/snakes stacked on the same shelving unit, and I am sure the gecks will appreciate being less accessible to the cats!
Editing to add: I live in wildfire country and have actually been evacuated three times in my life due to wildfires, so I am SUPER paranoid. I'd recommend getting a Fire Extinguisher Ball on Amazon and keeping it near the snake shelves. The ball can be thrown at or rolled into a fire if needed. It is filled with a fire retardant material and if it comes in contact with extremely high heat, it will automatically release fire supressing material that is harmless to humans (unsure about animals, but I've heard it's safe. I haven't verified that, though). I have first responder friends that swear by them and I've kept them in my kitchen and in my snake room since I learned of them.
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