Ripping off tourists is the national pastime.
do they not like foreigners?
Not really. But they put up with them because of the $$$
should I expect people to try to rip me off?
Yes, especially in Phuket or any farang-dominated area of Thailand.
I would avoid not only tour guides but also "tour companies" since they are often in league with organised scam/theft rings and at the absolute best you'll pay more for something you could book yourself on the internet.
The best way to find out about Thailand or any place in Asia for that matter is to plonk yourself in a pub that's got a few expats in it and buy them a beer.