The bedding pellets should be fine. Horse stall pellets are safe for rabbits. You may want to open the bag and let it air out for a couple days before you use it.
I think that Manna Pro is a decent brand. It is alfalfa based and tends to be higher protein that what most pet rabbits need. You should keep an eye on your rabbits to make sure they aren't gaining a lot of weight and aren't getting other issues from the alfalfa. You could mix it with the stuff you are feeding so they still get some of the benefits of the timothy pellets. Personally, I would not change the feed unless it is not working for some reason.
