My son always suffered from constipation and the simply thick never made it worse. For babies who have regular bowel movements (i.e. they have never been constipated), it may make the stool a little more firm, but trust me, as constipated as my son was prior to the simply thick, if the product was constipating, Luke would have been miserable.
AS for the nipple size, I honestly can't say I've ever had a preemie nipple, but certainly, sticking with a smaller nipple size will help.
With your son being 7 mos., I would start encouraging a straw cup and get him off the bottle ASAP. Start by plugging one end with your finger and giving him sips and he should figure it out within a short period of time. My DD figured out straws at 5 mos. and though Luke wasn't quite that young, it was well before 1 year with him.
Good luck at the doctor's appt tomorrow!