I know that angelfish get very big. But I want to keep them together for some time. My problem is, can Glofish do any harm to such a small angelfish? I want to keep them with only for a month, as I do not have a big tank yet.They will become much much bigger. For a 20g by themselves you could do 2 mature angels tops and nothing else. I personally wouldn't do it unless you have another larger tank and looking for a pair of angels to breed, then get 6 juveniles and you'll likely get at least 1 pair but they would need to move to the much larger tank and then the pair alone in the 20g. The mating pair will be too aggressive to be with other fish.
The link you provided are altum angels. Those guys will push 8" diameters at maturity... just not a good fit long term for a 20g.
Thank you for helpThe glofish won't harm them, and the angels won't harm the glofish until they are larger. The size they are at is approx. 2-3 months old. They will reach quarter size at about 3 months and by 8-10 months be about full size and mature and finished growing at 1 year.
So the clock would be ticking for space within 3 months of getting them you'll have to do something to get them more space or risk them getting stunted and mishapened.