to consider Phuket, rather than Koh Samui. Samui is really pricey with extra cost for the airport anytime you fly in. (Edit: just read the post above about Samui resident airfares, so this argument is not valid any longer!!) The north side of Phuket is still very green, and there is a large, multicultural expat community (many of whom are long-termers).
I'd recommend taking some time to stay in each location and chat up some local expats to see which you feel most comfortable in.
It sounds like they thought you were applying for a nonimmigrant B visa, which is for working. In that case they need the marriage certs and birth certs for the dependents' visas. Only reason to ask for those docs, because tourist visa doesn't require them. So maybe the staff just didn't look carefully and assumed, since many people just get the automatic visa on arrival and extend. Just a guess, but the only thing that makes sense from my experience (lived/worked in Thailand and travelled there for visits in between since 1992).