It totally depends on the nature of those conversations.
The point is, people seem to be commonly regarding the mere act of talking to the Russians as both proof of guilt and as being something inherently illegal. It's extremely far off base from a legal standpoint (that's the witch hunt aspect that has been present since Hillary lost, revenge lust, etc). There is a lot to prove yet, even if Trump did something illegal.
Is it illegal to talk to the British or Israelis or Japanese before election day? I mean, comeon. It is not illegal to "talk to the Russians." Russians are not some special illegal class of humans.
What matters is what actions were directed or not, etc. For example, did Trump knowingly instruct Flynn to break the law, can that be proven, did Trump have knowledge of something illegal going on (even if he didn't direct it), what was it, and so on and so forth.
Are you saying this : https://en.wikipedia.org/wiki/Logan_Act doesn't apply?
People should remember, Nixon wasn't brought down by Watergate. He was brought down by an attempt to cover it up. The way Trump can get impeached, is if he lies. Ordering Flynn to talk to the Russians (eg to see what they had on Clinton or similar), won't get Trump in anything even remotely close to enough trouble to get him impeached.