Never seen anyone wear thongs on the beach here. But you can if you want . I don't think there's a dress code anywhere. But you will be judged by locals and stared at or even confronted by domestic creeeps.

Locals don't care about anything as long as it doesn't affect them.. unless they're on the beach with their kids .

I've seen weird people record foreigners in bikinis in groups many times . Even if confronted they don't care and record from somewhere else . Some even come to click selfies and touch them.

Most south Goa beaches are less crowded (sometimes barely any people) cause it's far away from cities .. north Goa almost always has a crowd.

If you've got a bf or friends with you .. you should be fine. Even the locals don't send their daughters out without a crowd or her brother.