To answer just one of your many questions, your chances of getting into grad school really don't depend at all on whether you major in physics, finance and physics together, or finance and physics separately. They depend on your grades, your physics GRE score, your research experience, and your letters of recommendation, and not necessarily in that order. Having a broader experience with outside coursework or degrees can only help, it can never hurt, as long as you demonstrate a focus and an aptitude for physics.
Now, if this interdisciplinary degree only takes one extra year, are you skimping on any of the core coursework in physics or just the electives? It will hurt your chances if you are perceived as not having the strong physics background necessary to be at the level of the other incoming graduate students. However, if you complete the major core courses (upper division mechanics and E&M, quantum, modern, and thermodynamics) and score well on the physics GRE you should be in good shape. If you are going to take these classes anyway to prepare for the GRE, you might as well get the recognition in the form of a degree.
Another bit of advice, hook up with a physics professor doing research that interests you. Do this right away, and do research from now until you graduate. Try to get a publication or two. If there isn't a professor with an opening, get an intership at a nearby lab. If none of this works, apply to REUs for the summer. Just make sure you do research! This is hugely important when applying to the top programs, if that's where you're aiming. If not, solid letters of praise from your research advisor can make up for a lot.