I've owned two JD 750's over the years but neither one had a loader. I thought about adding a FEL to the 4x4 model 750 that I had, but decided against it. The main reason I decided not too was because the 750 does not come with power steering as standard, and adding it would be about $1000 or so I was told by a JD dealer. If your 755 has power steering, then you can add a loader and get maximum use with it instead of fighting the steering wheel with a fully loaded bucket.
I'm pretty sure the 755 takes a Model 70 loader, same as what's on my 790 now. However, you cannot purchase a new Model 70 loader from JD because they don't offer them anymore. If you really want one, you have to get a used one (very hard to find), or you'll have to get an aftermarket brand such as Bush Hog, Koyker, or Westendorf. I know the Westendorf is Quick Attach, and I think the Bush Hog brand is as well. Not sure about the Koyker though.
I don't know if any of those three brands offer a 4 in 1 bucket for their loaders but you can check them and see. One thing you will have to consider is that the Model 70 was rated for lifting slightly over 800lbs (on the 755). The 4 in 1 buckets obviously weigh more than the same size standard bucket so if you can find a 4 in 1 bucket, whatever extra weight it adds will probably take away from some of the lifting capacity of the loader. Good luck!