I know a lot of them reside in the Northern part of Thailand, and a lot of them have learned Thai, but I would prefer ones that had some experience in Burma. I'm willing to learn their language and I know it won't take me long, because I have learned a decent amount of Khmer and I know Lao.

So, where is the best place to meet Karen (or other ethnicities from Myanmar) in Thailand?

How open are they with someone who speaks Thai?

Go to Mae Sot, on the Thai-Burmese border. Elsewhere they may be more difficult to spot and wouldn't be present in large numbers in any given location.

They would be more than happy conversing in Thai, if they can speak Thai. However, many of the Burmese (including Karen/Kayin) in Mae Sot and surrounding areas can't speak much, or any Thai at all, particularly if they are refugees.

I doubt very many would be able to speak English, so Thai, Burmese or Karen would be the languages to use with them.

That's fine

Maybe there's a Burmese language school somewhere in Mae Sot. I can try that out

try the hidden houses of ill repute in chiang mai

Why would I want to meet one of those disease ridden ones when there are plenty who aren't? I'm not looking for sex but something more long term