Darf gouramis aren't particularly hearty, at least mine wasn't... they are more of a man-made fish which makes them more susceptible to diseases. From my experience, corys are very hearty and they live a long, long time. I have no experience with platys but I do have experience with swordtails which are closely related. I still have the swordtail from my very first tank which was a disaster! two out of my first six fish made it, and he was one of them. He lived through very high levels of ammonia when I still had no Idea as to what I was doing and a long move, so I would assume that platys are hearty too. Just make sure that when you buy a fish, it looks healthy to begin with.
if you are planning on keeping cories i would suggest you thoroughly research any species you are interested in....some prefer warmer water and some very much cooler than normal..species such as Sterbei prefer it in the low to mid 80s while Barbatus and some others like it in the mid 60s to low 70s......many of the aspidora species also like cooler water..
platies are pretty hardy little fish and readily available in quite a few color variations..
dwarf gouramis are another fish that likes temps in the upper 70s to low 80s...while originally from southern asia it also comes from areas like bangladesh and pakistan..years ago it was called Colisa lalia , but is now called Trichogaster lalius...they keep changing the names on me....i hate that...lol
unfortunately most are now coming from singapore...many of these fish carry the IRIDOVIRUS.....if you find some good healthy dwarfs buy them..they are beautiful little fish..
I used to keep platies. I found that the ones I bought in my Lfs weren't very hardy but they still lasted about a year. The ones I bred off them were more hardy and lasted about 2-3 years. I had dwarf gouramis which were hardy and lasted through some rough conditions when I had a problem with my filter but they became quite skinny. I have never had any corydoras though
im 13 and have a 34litre aqua one tank which has:
2x Botia striata
1x tiger barbs
1x red tailed black shark
1x guppy fry
95litre marine tank
4 emerald crabs
1 hermit crab
1 haloween crab
2 sexy shrimps