Hello here
And welcome to the forum! I would suggest taking a peek through the puffpedia at the top at different puffs and their requirements.
As to your questions:
1. 15g is not big enough for 2. Just about big enough for 1.
2. These puffs really do well in groups especially of 4 plus, this is one of the pufs that is not recommended for singleton.
3. Oops answered that! The general results on here have been they arent very happy single.
Here is a link:
ug.php/v/PufferPedia/Freshwater/C_Asellus/
These are one of the puffs that also require regular teeth trimming, and prefer long tank length so the larger and longer the tank you go to accomodate a group the better.
That tank would be ideal for dwarf puffers who are chock full of personality. A 15g you could have 3-4 puffers.
ug.php/v/PufferPedia/Freshwater/C_Travancoricus/
If you could be tempted to maintaining a low brackish (salt tank) you could have a singleton Figure 8 in there:
ug.php/v/PufferPedia/Brackish/T_Biocellatus/
Or even if you could upgrade to a 20g - 25g the world of possibilities really would be opened up with red eyed puffers, red target group puffers, etc.