Welcome to pufferdom! Puffers are excellent pets and really intelligent. I would really suggest having a species tank only to be honest as most puffers tend to not tolerate other species or even their own kind at times. There are some species that are considered "community" safe but it is always going to be a risk. As to a 180 litres, here are some suggestions on basic info, I would suggest looking or asking more specific questions as to upkeep etc as some have varied needs from others. First of all are you wishing fresh or would you be willing to go slightly salty to brackish?
For suggestions where mention more than one puffer, this is based on them growing together, the puffs personality indivudally and also tank being heavily decorated/planted - line of sights broken
Here are some suggestions:
Figure 8 Puffers - 3 inch adulthood - require low brackish fairly easy to keep and some have had success keeping with bumblebee gobies, knight gobies and others with mollies! Could try 3 together (4 would be really pushing as need good filtration as well)
GSP (green spotted)- these require higher salinity levels and some take to full marine eventually, mentioned depending on what you are after. 6-6.5 inch adulthood. Would say one, two possible but pushing
Red Eyd / Irrubesco - these are good if can get male/female groups and ratios, freshwater, get to 2 inches roughly. As to numbers not excellent with these though from reading bigger tanks better for them and a male and two females wouldnt go amiss, these have been mixed successfully in communities but pick occupants with care (that wont be nipped or in return wont bully).
Dwarf Puffesr - freshwater and get to an inch, these are tiny but mighty and a good first puffer! They require about 5g per puff though some work on 5g for first and 3g for each additional, again with these need 2f per 1m. Could have about 10 puffs working on 5g per puff rule. I have read interesting things on these in larger groups as generally people try 1-4. People have also kept successfully with otos and also communities but again depends on longevity and personality etc.
SAPs (south american puffers) - extremely active, get to 3 inches, need to be kept in groups to keep happiest they need good flow as always on the go. With these be aware that most have to keep regular teeth trimming on these guys even wth regular crunchy foods in the regime. These have been kept with other species of fish but they are pretty nippy. Freshwater
Abei - need to be kept by self. Pretty active fellows and minimum tanks are 95 litres, get to 4 inches. My fellow is cheeky, depends if happy with one fish in that size tank. Freshwater
Baileyi "hairy" - singular get to 5 inches, freshwater. Tend to lurk abit though mine is pretty active again, interesting and quite rare depending where situated.
Palembang - freshwater, singular, get to 8 inches, pretty inactive though a larger species.
Hope that gives food for thought, I have had a fair few species by now (with a lot of help on this forum) and settled myself on abei, fahaka, baileyi and miurus in their own tanks. I have tankmates with the fahaka but this is my own risk.
Have a read, google the images, see what you can get in the area and what takes your fancy!!
In terms of food, most of the species mentioned above are easily trained to frozen food including bloodworm, cockle, mussel, prawn, oyster, squid, etc Some takes more patience than others and some take to any food straight away. Flake will very rarely be taken and not appreciated. Setting up a snail breeder tank is always a good idea for free crunchy food, ramshorns or pond/bladder snails will do and if regular water changes are done then no need for filter and heater depending where placed in the house.
Hope havent scared you off but a bit of a fan when it comes to puffers
