Strongly agree. It'll be very tough to find a free chat that not only offers the specific qualities that you (addressing the OP) seek, but that would also be open to inviting someone of unknown/unproven value. You need to look at it from the standpoint of those already in such a chat -- why should they invite a stranger into their chat when they're doing fine already? Almost inevitably, we all know that a growing number of people in any type of group eventually leads to conflicts and discord.
The only chat rooms like that which I'm aware of started out in already-focused, value-added *paid* chat rooms, where subs generally traded the same style (as taught by the service). From there, smaller groups of traders occasionally broke off and started their own free, by-invitation-only chats -- often because they became disgruntled or disenchanted with the original room/moderators/service (e.g., too much noise, increased prices, personal disagreements with mods, etc.). And typically, such members already had fairly well-established relationships, as well as "proven" track records to some extent. At the very least, they had already demonstrated that they could work and get along with the others.
I suppose a smaller, "breakout" chat could also be started from an established, *free* chat -- but it might be much harder to winnow things (/people) down, because of all the diversity, noise, etc. that typify free chats (paid chats already provide a filter, as
@Sprout noted)... and then convince them to form a separate chat with you. If that's the route the you decide to pursue, then I'd suggest first trying to establish and build friendly relationships with like-minded traders you see via private chat (DM)/emails. Eventually, you can grow your trading relationship(s) into a "formal" group.