[@Bloodwolf 2] They'll be encountered throughout the RP. Some have known locations, others don't.

And yeah, being a Traveller means he can't advance to Myrmidon. Taking that class mean you'll skip that tier of classes and go right into Expert, but you require the same requirements for that. Basically it's a tradeoff to be a bit stronger early while giving up a power boost or benefits the class may give. This does mean you may miss out on stat ups, so be mindful.

I wholly recommend Traveller if you plan on making use of it's ability to ignore terrain though.