So I've learned that due to the way the character selection RNG works you won't be able to pick the same bro until at least 27 other rolls (assuming you've unlocked all characters). I'm thinking of making a program that'll monitor what bros you've already rolled and tell you what characters are actually possible to roll. I'm just wondering if something like this should be against the rules or similar programs?

That's stepping into the realm of modding/third party programs which are generally not allowed. Although there is some room for discussion still since communities can decide to allow such tools.

But right now it would be a futile endeavour as everyone is waiting for the next large update which is supposed to hit soonTM. Recently a presskit was leaked which had info on that update and it included a few new bros too. Alongside code changes which will likely impact your programs functionality (which is also why there currently is no LiveSplit ASL support for Broforce) it might be better to hold off.

You could of course test out if this is even possible.