Hi Matt,
Welcome to TONMO.com
Firstly, if you are in the States like most of the people here then getting cuttles in the first instance is going to be a real pain. If you read back some posts in this forum you'll see what problems other people have had. The species most commonly sold in the USA is Sepia bandensis and not entirely suitable for the home aquarium. It is normally imported as an adult at three - four inches and doesnt travel well. Many people think they are cyanide caught too. So either way many dont live more than a few weeks.
Luckily for Mike and I, in the UK s. officinalis is reasonably easy to get.
So, here's your questions....
I want to ask if aragonite is okay for juvenile cuttles and if cuttlefish would prefer the less vast yet natural environment of a reeftank or the vastness of an empty tank environment
Well, aragonite is fine for a cuttle tank. However, as Mike says, they do like to bury themselves, especially when young. So i use a mix of 50/50 fine aquarium silver sand and either aragonite/calciumplus/aragonite to help stabalise the pH. this fine sand prevents any harming of their delicate skin.
The size issue is the real issue here and always will be with cuttles when trying to figure out reef or bare tank.
I have done both, if you look at these pics of a tank i had then you will see that they (7 of them) shared their home with corals etc. However, that was a system of just over 200 gals (UK) So i had a bit of space to play around with decor. Once the cuttles got to over 6" long even that was too cramped for them and i had to whittle the numbers down to 2. When the last male was by himself in the tank at over 10" i feel that even a 200 gal tank was too small for him. So your question about how many cuttles to keep in a 40 gal tank is somewhere between none and one. Thats asuming that the species is officinalis of course as a couple of bandensis would be okay in a tank that siz or officinalis up to about 3 inches.
Their is not to omuch other info available in books or on the web about keeping cuttles but several people here have so its probably the best place to ask any more questions in the future.
Thanks for your questions and here is a link to my cuttle pics
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Cheers
C