The gemara says that a shomer may stipulate to have a lower level of responsibility because מעיקרא לא שעבד נפשיה, meaning that his original acceptance of responsibility was never for more than he accepted. Tosafos in Kesubos 56b asks that this seems to undermine the entire concept of stipulating against the torah. Why can't we say the same thing for one who marries on the condition to be exempt from feeding and clothing her is also מעיקרא לא שעבד נפשיה so he should be able to make such a condition? Tosafos gives 2 answers: 1. Since the Torah doesn't only list one type of shomer but rather 4 types of shomer, we understand that the intent of the torah is to allow anyone to accept responsibility like any of these 4 shomrim. But by marriage where the torah doesn't offer different types and packages, it is only binding if done in accordance with the package that is recognized by the torah which includes she'er k'sus and o'nah. 2. Since the Torah obligates a borrower more due to the fact that "all the benefit is his" and a paid watchman is more obligated than a free watchman because he is being payed, indicates that each shomer has the right to accept upon himself whatever level of responsibility that they feel they are being compensated for. Meaning, the ultimate responsibility of a shomer is a direct result of what he had originally accepted upon himself, therefore it is in his control to limit what he accepts upon himself.
Perhaps there is a distinction between the 2 answers of tosafos as to whether a shomer sachar can stipulate to be chayev on aveida but patur on geneiva. Based on the first approach we may only allow one to stipulate to be like one of the categories of shemira that is already recognized by the torah, but based on the second approach one should be able to create their own category.
Rashi seems to answer the question of tosafos very simply. Some interpret rashi that the reason by kiddushin one cannot stipulate against the torah is because the moment he says "הרי את מקודשת לי" the kiddushin is fully binding and no longer in his control to stipulate על מנת שאין לך עלי. But this peshat in rashi is very difficult as Tosafos in kesubos asks, the stipulation must proceed the ma'aseh. If not for Tosafos, I would have understood rashi to mean that kiddushin is an entity. Meaning it is a "chalos" that is designed by the torah. In essence it is a imaginary concept that only exists based on the torah design. Therefore, to stipulate and change the design isn't possible and completely beyond one's control. But a shomer on the other hand is not a "chalos", it is merely a list of responsibilities, therefore prior to accepting those responsibilities one can choose which of them he is willing to accept and which of them he isn't willing to accept.