Improperly handled racing colts are nasty SOB's (pushy 5yo standardbred stallion, anyone?), but it sounds like these guys have had the right handlers. I used to work in a racing stable that had quite a few older colts/stallions that were still racing. However, from what you've said about these colts, you may find that 'NIFTY' will start to assert his dominant side in an unwanted manner if you don't continue to handle him in the same manner as in the racing stable. It's a lot harder to comment on 'BRUMBY,' as shy colts can either remain shy when away from a yard full of horses, or he may discover he has a voice when he's not being told to shut his pie hole by everyone else.
Being stabled together means nothing - most racing stables don't have the space to spread out their colts between the geldings. However, it may mean they will be much calmer around mares because they're used to them bouncing around the joint. If you want to see one of the worst examples of how bad a stallion can really be, do some research on the Sir Ivor stallion, Sir Tristram. He threw the studmaster's brother clean over a fence, and was a notoriously tricky horse to serve (if he didn't like a mare, well then, he did not like that mare and various tricks like covering with his favorite girl's scent or using a teaser mare had to be used to get the job done). That said, Sir T was, and still is, one of the most influential stallions in the southern hemisphere.
On the subject of their potential, there's not much I can say. I would suggest giving them a chance, since their conformation sounds acceptable. If, say, one of them turns out to be a spectacular jumper then consider
keeping him entire. If not, then remove the offending part of his anatomy and find him another job. I could definitely see Brumby going into polo once gelded, if he's got a nippy bit of speed to go with his size and shows confidence under saddle. As you probably already know, TBs can do anything. However, if they can't do anything in a spectacular way, or can't prove themselves worthy of the extra handling and specific environment required of a stallion, then I say geld them both.
FYI, when I said give them a chance, I don't mean just a week or two. It's entirely up to you, but I might give them a bit of time to come down from race fitness (and whatever drugs they've been given) then see what they can do. You might want to give them a six month run then bring them back and test them. Either way, you should be sure they're going to amount to something before trying to market them at stud.
Also, I love Nifty's pedigree, but I'm a sucker for Danehill and his sons, especially Doubty and the Rock. It's also no surprise he's got a short back with that sort of pedigree, and if I might make a suggestion, give him a go at jumping - I know a few people who also have Dane-bred eventers and jumpers that did well, though Grosvenor eventers are more common in NZ now.
Yes Stallions get more stallion like with age and will test more and more. They need strong consistent training. These guys are just babies they will become more confident with age.
As for that comment, well, maybe you should learn to count as these colts are four year olds, not two year olds. Thoroughbreds are fast maturing horses, physically and
mentally. Handled consistently and firmly, they should be fine temperament wise.