it should say that males MAY become aggressive when they are 5-12 months. essentially this is the teenage years and like humans teenagers are rampaging with hormones an it makes them act a little odd sometimes. also, they should have had a note on the word agressive itself. the type of agression does not mean that WILL be fights it just mean that they are more likely to happen now (just like human teenagers). it also does NOT mean that the fights WILL BE serious, again just that there will be more. a fight shouldn't be anymore serious then normal rat scufflings.
however, having said that there CAN be rats that get quite aggressive and will seriously fight (though not normally with other cage members that they have grown up with). if this happens a neuter will most often fix it but its not terribily common that it will progress this far.
so to answer your questions, yes, in most cases you can keep them together and in most cases they won't get into serious enough fights that could harm each other. but, if they are the odd ones that do neutering will more likely then not fix the problem.