Maybe switch the big fish to the 10 gallon and the fry to the smaller tank - not sure how to do the water change since it sounds like the mollie has a fungus if furry.
Making an emergency QT is also an option. It can be very inexpensive, just don't use any materials that have been washed with soap. Maybe you could get a larger plastic storage bin as a temporary solution for the tank that is way, way too small. Maybe switch the filters and use the 10 gallon for the temporary large clear plastic container which will bring beneficial bacteria with it, though may not be cycled entirely. Maybe someone else can give a good suggestion about how to keep the temperature appropriate.
You really, really need to get the 2.5 gallon fish into something way bigger. Even one goldfish needs at least 10 gallons.