hi, teracotta pots should be fine as long as they are new, and have never been used for plants. I like to boil anything that gets added to my tanks as well just in case.
As for sand, yes it does get sucked up with a syphon, but unlike gravel, waste does not sink into it, it simply rests on the top, so it is possible to hold the syphon above the sand and the suction will lift the lighter waste off the surface of the sand.