yeah, variety is best tho, also use crickets, algae wafer, un cooked shrimp, tilipia fillet and boiled shelled peas!-)
To be honest mate just keep what you want in your 75, the o will grow faster so i would advise gettting them both at around 5-6 inch so the jd doesnt become a risk, have a divider at hand and its worth a shot imo if you are willing to keep up with the maintenance. at the end of the day, the fish could and most will probably end up in a much worse place than an mfk members 75g.
i often find myself saying what i know is right considering fish's max sizes ect, you will know what is right and if it starts looking tight im sure you would upgrade if need be, get the birchir aswell if you plan to upgrade in a few years!-)