How do you handle in every day life, at conferences, in hallway conversations this paradox of collaborating and competing at the same time in academia?
It is very difficult indeed. Motivation for pursuing a career in academia vary significantly, and accordingly, what is considered as ethically acceptable.
From experience, it is often the issue of the man in the middle,
which in practice is the major source of frustration. Individual A talks to Individual B about his ongoing (unpublished) research. Individual B then more or less forget about where he got this information, and speaks to Individual C, who implements it, unsuspectingly. Everyone behaves ethically at his level,
but globally, Individual C effectively can be perceived to compete aggressively with Individual A.
A solution that some people seem to adopt in conferences in my field is to only present/discuss published material, which makes attending conference less interesting, as it only involves outdated research. Another sub-optimal approach is merchandizing, i.e. present one's research at a superficial, advertising level, so that the actual real issues/breakpoints are effectively not discussed.
On the other hand, research thrives in confronting honestly different perspective on a given typically complex problem, so there is a lot to be gained in collaborative behaviour.
Modern research is also fairly specialized, and conferences are the one place where you are likely to meet experts in your field who have given some thoughts to the problems you are interested in.
In the end, everyone has to balance these things out.
My advice would be to behave on the cautious side, but then again I tend not to follow my own advice. Another approach is to make sure you are so much on top of things that it does not matter :-)
Do you avoid colleagues who are known to 'tell only the minimum' at congresses and then surprise later with an accepted grant?
Well, life is short, so interact preferentially with colleagues whose motivation for doing research seem to overlap most with your own.
Striking a balance between collaboration versus (unrestrained) competition is not specific to research/academia in fact. It is the basis of civilization! What is a bit specific to academia is that it is (poorly IMHO) self-regulated. There is no such thing as academic police/justice. I found this RSA Animate to be instructive to get a measure on how a small amount of policing in enough to get the system working.
Another point worth mentioning is that predatory behaviour is in fact not that common, if only because people are too busy with their own train of thoughts, and also because it takes time for new ideas to percolate. To understand why a given idea is novel, typically requires having spent some time thinking along similar lines.