No - online casinos have very little impact on how their slots actually perform. The slot provider sets the RTP value and controls the backend and results of the slots you use. The casino doesn't get to dictate what the winnings for individual slots look like. On the other hand, some casinos do offer their own proprietary games (i.e. they develop them in-house and don't licence them from Netent, Microgaming etc.). In this case, the operator will set the RTP value and control the backend, but their regulated status should mean that the results cannot be rigged.