Well Superman, I can tell you when it comes to puffers the size of the fish doesn't lessen the enjoyment to their caretaker.
I have a pack of 6 Indian Dwarf Puffers (they are a true FW Puffer that grows to 1 1/2 in.) housed in a 25 gal Hex. tank with 4 Bumblebee Gobies. Now I know the BBG's will have to go to a BW set up eventually, but they are doing well now and they are the ONLY fish I have been able to keep in with The Tiny Terrors.
Now don't get me wrong, I Luv my babies
, they are the reason I'm a fish keeper with MTS (1-90 gal, 2-35 gal, 2-25 gal, 1-10 gal, started Dec 25th 2002
), but they are little Demons when they want to be (long story, I'll post it here if you want to read it). I find they are more intelligent than (FORGIVE ME CM et al) any of our Cichlids, and also have more personality. I have even watched them sulk for 3 days anytime my wife walked into the room because they were mad at her. (look what you've done...he's off on a rant now...groan
)
Now that I've frightened you, and possibly intrigued you, I would recomend the Dwarf Puffer for your 15 gal whole heartedly. You could get probably 3 females & 1 male. Just be sure to have lots of caves/plants/holes for them to hide and sleep.
If you want to contact a few Puffer experts (people with 20 & 40 years as puffer caretakers) try
Aquaria Central and look for "Pufferpunk", "rjl420", "andruboz" or "RTR" these four probably have close to 100 years of personal experiance owning puffers off all kinds.
Oh and in answer to your original questions:
I would like to know what the largest true fresh water puffer is, and how much i should expect to pay for him
MBU is indeed the biggest FW Puffer. I'v read recomendations for no less than 500 gal tank...single fish.
I would like to know if these are the type of fish that will just keep eating and eating if there is food available
Yes, some puffers will literally eat themselves to death (Dwarfs incl.), they will eat so much that their bowels will become obstructed and result in death
Can I keep more than one together? Do you recommend a species tank?
Yes you can keep more than one together, depending on species (Fahakas for example have been frequently discribed as "A fish on crack that will attack anything that comes into their line of sight"), but in a 15 gal tank you are limited to a smaller species, and most posts I've read recomend a species tank(although many other Dwarf caretakers are surprised by my packs escapades, a lot of people report theirs a "good community fish"...time will tell).
Well I've rambled on long enough (more than long enough...
), hope some of this helps.
Don