The easiest route is the s/a option. Generally you can keep a mix of geos or Severum,festivums chocolate cichlids etc without to much aggression.
A group of geophagus and a few festivums would make a nice tank and also gives you an option of adding other non cichlid tank mates such as characin and catfish.
You could also have the option of plants in this type of set up.
A south American biotope is definitely the set up that gives you the most options.
Personally I don't like your second option of mixed c/a and s/a cichlids.
People do mix them and often it works out fine if the right candidates are chosen and the tank is big enough.
However, to me personally it just looks wrong.I like to see fish together that could be found together in nature. A mix and match of fish from all corners of the globe just does not do it for me.A lot of these fish will have different preference for ph and hardness.They will often have different levels of aggression and territoriality. Another issue that can often be overlooked are differing temperature requirements.
Like I said before,people do mix them and make it work,so I don't want my opinion on this to put you off entirely if you wish to try.
Your third option of a mixed c/a tank is probably my favourite option.
You will be limited on what species you can keep together you will be limited on non cichlid tank mates and depending which species you choose,plants may not work either.
I do however think the cichlids from c/a are often the most attractive, have lots of character and are well worth the trouble they can sometimes bring.
In a tank your size I would avoid the big notoriously aggressive species such as jags and certain amphilophus species.
There are larger species that are not always quite so brutal as others.
Veija melanura are usually quite laid back and can make a good show fish amongst smaller species like cryptoheros or groups of thorichthys, Rheoheros or astathoheros.
The last three of these cichlid families all do well in groups of six plus.You can keep your larger sized show fish with these smaller groupings and still observe their breeding behaviour and interactions with the rest of their group.
Another large peaceful c/a fish is petenia splendida. It is a very predatory fish but anything too large to swallow is usually ignored.
They do get rather large so tank size may be an issue.
Jack Dempseys can also be laid back if not housed with a mate.They can also be quite nasty.
Central Americans can be trial and error no matter how carefully you try to stock.
There are fish more likely to work than others.
In a tank your size I would avoid any fish that have a bit of a rep for aggression, they will more often than not live up to it.
Here are a few species more likely to coexist in a community type setting.
Thorichthys,
There are around ten species to choose from, All do well in groups are colourful and relatively peaceful
Astathoheros
These are also colourful relatively well behaved do best in groups and a very interesting fish to keep.
Cryptoheros
These fish do well in pairs or harems. They are great little cichlids,full of character,attractive and generally peaceful enough for life in a community setting.
Rainbow cichlids
Probably the most peaceful c/a cichlid available. Again they do great in small groups and are easy to keep. May not tolerate bigger more boisterous tank mates as well as the cryptoheros family.
Hypsophrys nicaraguense
Another very attractive cichlid that could be included in a c/a community.
Veija heterospilus
The smallest and probably the most peaceful member of the group.Very attractive fish that do well in groups or harems.
Should work well with a small group of thorichthys and a pair of cryptoheros such as spilurus.
Non cichlid tank mates could include mollies,swordtails or large goodieds. Astyanax tetras would also be a biotope correct option if you can source them.
Non of these non cichlid fish would last too long with predatory c/a species.I would get yourself a list of species you like,research them well and ask again on this forum before you commit to buying them.
Good luck and hope it goes well.