It is tricky but you can actually mix and match fish. I actually have a 7-8" texas red cichlid that I bought when it was only about 1/2". I was able to keep that fish and a fuco cichlid in my 60 gallon that housed my tropical fish.(they were too small for the large tank at the time) Those fish were fine with the tropical semiagressive tank mates even when they grew to a large size. A year later we moved and I gave away a bunch of cichlids, but kept some only to have them fight like crazy and I had to bring all but 6 to Davie Jones to find new homes.
Anyway back to topic I would still say try freshwater first. If your still into it after a year sell the setup and go saltwater.