It depends, some fancy goldfish can withstand tropical temperatures up to 24 Celcius and can be adapted to that, and many tropical fish thrive in 24-25 Celcius. The second point is food, the foods that are made for tropical fishes hava a higher protein content than goldfish food, but I am not sure to what extent that may affect their diets, based on experience and some lectures it is not detrimental. Third, space the size of your tank and the decoration are very important the more space the better, in a typical 50 gal tank I wouldnt recomend it, but if you have a larger body of water say 250 gal+ it can be an option. Fourth a very good bio filtration in necesary, an over capacity larger filter is better. Fifth and last I am not sure about guppies but Mollies like to pick on the fins of other fishes, and a pearlscale goldfish has many, so you will need lots of space and hiding places as distractors.