Yes you can layer vinyl...just be sure you use calendared vinyl on calendared vinyl and cast vinyl on cast vinyl.
651=Calendared
751=Cast
651 will be fine for what your doing, 751 would be preferred though. Whoever told you that it will take off paint does not know what they are taking about. If your paint job is from the factory you have nothing to worry about. We put 651 (more often 751, but they have the same adhesive) on expensive cars/suvs and never once has there been an issue with the paint coming off.
(dont use 631, it doesn't use a permanent adhesive)