Hi Mike,
I have heard that claim before but people who have had rats from breeders have reported that they were not healthier then rescue rats - some were actually less healthy. (Since this is a rescue based forum that does not support breeding that is all I will say.)
I would suggest that you get rats from a rescue or shelter or from someone rehoming their rats or rehoming an accidental litter.
If you want to prevent the most frequent types of tumours and aggression in males (and if you have access to a good knowledgeable rat vet) you will want to get your rats spayed or neutered.
http://www.ratshackforum.com/forum/showthread.php?t=7691
Female rats have a more the 85% chance of getting mammary tumours so people with access to a good vet knowledgeable about rats will get their rats spayed.
I have never heard of girls spayed by 4 months of age getting mammary tumours.
http://ratguide.com/health/reproductive/spay.php
btw girls in heat will chase one another etc
Rats can get other types of tumours but it is less likely.
A good healthy diet, good care, and prompt vet care can help to prevent or minimize health issues.
Males can be stressed by high hormone levels and if this occurs, people with access to a good rat vet who knows how to do surgery safely on rats, will get them neutered as this will solve the problem.
http://ratguide.com/health/reproductive/neuter.php
Double rex rats and hairless rats have more health issues then other rats because of their lack of hair.
They are also prone to eye issues and infections.
Rats with high white markings (white on areas that would not normally be white) or from a high white litter (parent or sibling have high white makings), may have a terminal (early onset) or difficult to manage (late onset) genetic issue called megacolon.
http://ratguide.com/health/digestive/megacolon.php
Quote: "you will often see genetic megacolon in BEW (black-eyed white), husky, blazed, split-capped, tei-colored, odd-eyed, and other “high white” lines of rats."
If rats are in a large cage (min 4 cubic feet with a base at least 30 in by 18 in for 2 rats) with lots of toys etc they may be active at night and wrestle and play
but if they are fighting then there is a problem to deal with.
I hope this info helps