Standard quality flake,
pellets,
catfish pellets,
bloodworms - live, frozen or freeze-dried,
brine shrimp - live, frozen or freeze-dried,
daphnia - live, frozen or freeze-dried
and the ocasional blanched greens - zuchinni, lettuce, some cucumber
and also sometimes a mashed pea or shelled pea...
Any of the commercial tropical fish foods will be taken by all those fish.
The cories need something that sinks though and gouramies need to be able to get to the food as they are rather slow, shy eaters.
A varied diet is always best.
As for the quarantine tank, you can use any container (not necessarily a tank) and with small fish like yours you need not use anything larger than 5 gallons. Though 10 gallons is prefferable. You need a heater and airstone and a small sponge filter will come in handy. Leave the sponge filter in an established tank for a week before you need to quarantine a fish so that it is colonised by beneficial bacteria. If you don't use a filter in the quarantine you'll need to do daily water changes and monitor water params. Don't put any live plants or inverts in the quarantine tank. It can be bare-bottom or have a thin layer of gravel. A few plastic plants and/or a ceramic pot are good hiding places.