I've been following this thread because I have no idea how to start conversations but I think I can answer this part of your question. Think of two people with their interests and knowledge mapped in a Venn diagram(you know with the overlapping circles) Certain things fall into the overlap between the two circles....that's what you're allowed to talk about because both people can hold up their end of the conversation. You are allowed to throw in parts that aren't in the overlap to test the waters otherwise you'd never know what to talk about. You just don't want the conversation to be one sided, you talking and them listening.

So you've got a thing for mushrooms, mention them in a common way without their scientific name say hen of the woods or morels or something, mention that the season is coming up or that you had some to eat or something that brings it to a common level. People can talk about food pretty easily so that's a good place to start. If they show interest you might be able to go on a bit.