Solitary, CR 8 or 9?, Standard treasure (any more, and they use it to power their trip to Elemental Water).
There a good deal stronger in a fight than an efreet or noble djinn, but lack the latter's spell-like abilities (except for
wish granting) and can't fly, which is a very handy power, so I'm leaning towards Challenge Rating 8, although I have no objection to 9 if you prefer.
Advancement: probably - , but we could do treasure consumption for these too. Then we can have them turn into "adults" when shifting to the elemental planes.
I'd prefer regular Hit Dice advancement and leave the treasure consumption to the Lesser Well Spirits. The 7HD Djinni has a 8-10 HD (Large); 11-21 HD (Huge) advancement, the 10 HD Efreet a 11-15 HD (Large); 16-30 HD (Huge) which suggests 100-150% HD (size) and 150-300% HD (size+1) is the scheme to go for, which would be:
Advancement: Huge 14-19 HD (Huge); 20-39 HD (Gargantuan)
I was thinking Common and Aquan, and maybe Auran so they can chat with Djinn.
But come to think of it, the Int 14 Djinn speak 4 languages (Auran, Celestial, Common, and Ignan) as does the Int 12 Efreet (Auran, Common, Ignan, and Infernal). Well Spirits have super-genius intelligence (Int 20), so I'm tempted to give them
all the elemental and outer-plane languages, plus draconic and common.
Languages: Abyssal, Aquan, Auran, Celestial, Common, Draconic, Ignan, Infernal, Terran
We can cut out some if you think nine languages is too much - my first choice for removal would be Abyssal, followed by Draconic.