I could put one in my 40 gallon. It just has tetras and crap. But it's an ugly tank.I wouldn't. DP's are really little. I bet a 4 inch cichlid could eat one. If not, the 4 inch cichlid will likely be a 6 inch cichlid some day. The 1 inch DP will never grow. Eventual feeder
The tetras can all die lol. I got for free with my tank and have been trying to get rid of them but can't find anyone to take them.They run risk of being eaten. Also The DP can nip your fish, even the bigger ones. Don't let the size fool you. They can kill bigger fish. I think you are better off throwing it in your tank with the tetras but they might be harassed.
I would rather get rid of the tetras and get a bunch of guppies and mollys and have like 4 puffers. Think that will work?Leave them in there. They can be dithers for the puffers. You can put 15-20 dwarf puffers in a 40g comfortable IMO.