Welcome!!!
At least they can only get so big. 75 gallons is pretty spoiled
When you upgrade the size, obviously, you will need to upgrade the filter, heater, etc.
My only recommendation with your new tank is to grab a calculator and a figure out the largest filter, heater wattage, etc. that won't be overkill for 75 gallons, but will carry though to 95 or a 100 gallons - that's the stuff that kills the pocket book when upgrading. It may hurt to shell out the cash now, but at least you only have to shell it out once.
As for when they are adults, it's hard to say = I guess it depends on how well they get along when they grow out of their baby size. You may find you need to separate a bully, or keep a love-struck couple apart, or...who knows.
Best wishes!