The answer to this can be extremely company-specific, particularly in a company with a significant presence in the US.
Colleague A: 1 year younger in age than me and joined the company at a later time. It seems like I could use the desu/masu form without Keigo, but since we belong to different departments, different branches, etc., I'm not sure whether he is in-group or out-group.
In a cross-department meeting or a one-on-one meeting, standard desu/masu forms are appropriate. If you're both talking to a customer (or other outsider), humble forms are often appropriate for anything/anyone related to your company.
Colleague B: Not 100% certain about age and seniority, but probably the same age and seniority as me or my junior.
Stick with desu/masu, unless it fits into the last comment below.
Colleague C: 6 years younger than me and much less seniority. Should I use plain forms to talk down to him? I decided not to since I wasn't sure whether we were all in-group or considered out-group.
There are two places you will routinely hear 'plain forms' in a business setting.
- Coworkers who are extremely casual (depending on the corporate culture)
- Speaking down to direct reports. Especially first line managers will routinely use casual (or even fairly rough) speech with their direct reports.
Everyone else pretty much always uses at least desu/masu forms.
You choose honorific predicates depending on who you're talking about, not who you're talking to. Speaking to a direct report about the VPs/visitors upcoming trip might involve the plain-form of いらっしゃる.