I'll have to agree with Jenste. A dwarf puffer is just asking for trouble, the galaxies should be in a shoal of 6-8, male bettas are known to attack guppies and mixing betta splendens (unless they are all female, in a big group and a fairly large tank) is another huge risk. Glad the bettas were taken back - hope the store apologised for misleading you!
Seriously, we've all been there. I've had some shocking fish combos in the past as a result of being mislead or doing bad research myself. I now don't trust pet shops (even if I think they are good) and always double check on here and other online resources. I've also got an excellent book on general fishkeeping that has become my bible (Tropical Fishlopeadia by Peter Burgess and Mary Bailey, if you're interested).
If keeping a community, I'd rehome the puffer or get him his own 5-10 gallon tank. When I worked for a pet shop I saw dwarf puffers taken chunks out of small plecs, tetras, live bearers - you name it, they attacked it. They are good snail eaters but shouldn't be used for that - snail populations can be kept undercontrol with good tank maintainence and not over feeding and puffers are not generally community compatible.
How big is the tank? I imagine it would be big enough for a nice group of the galaxies.
How is the guppy now?
What other stock do you have?
I don't mean for this post to sound horrible, it's just we've all been there and we're now fairly used to seeing new members get bad advice from shops.