It is not necessary to remove the fry from the mouth....Comercial breeders only do this to prevent any fry from accidently being swallowed or loss during their incubation....To answer your second question...Arowana are pretty much non-aggressive fish if kept in a large tank or pool....From birth, it is possible to keep all the fry from one spawn together...Given that the factors are met on space, filtration, and feed....I have seen a group of 30 "yolkies," or juveniles with yolks, grow to 4-6 inches in a 100g....
Your signature says you are from Brasil...You, among anyone, should know first hand on the proper raising and enviroment factors and needs of a Silver Arowana...I'm am pretty sure you are within an hours way from an Arowana filled stream or river....It must be amazing, takes some pictures for us.....