At worst, they'd be treated as another Catholicism. A bunch of sketchy foreigners out to undermine the country, not to be trusted, since the US was basically a white Protestant society. They'd probably be classified as undesirable foreigners and denied entry under immigration laws. But assume you have more liberal immigration laws, and a sudden spate of Muslim immigrants, I think the US would freak, but not any worse than when Catholics were mass immigrating. From everything I've read, Islam is portrayed rather positively in 19th century American sources, and could possibly be as equal as Catholicism was. Muslims are probably the best to diversify America, since Turks, Syrians, Lebanese (the non-Christian ones), Egyptians, and Maghrebis immigrating might be slightly easier to take than Chinese or Indians immigrating.
Hindus, well, that's India, and we once again have to ignore the fact that the US simply did not want Indians, and it wasn't because of their religion. I doubt most Americans will like Hindus, but they'll learn a cautious tolerance. Same with Buddhists, who are likely to be Chinese or Japanese. They aren't wanted, and it isn't because of their religion.
But I doubt Protestants and Catholics would make common ground on this. I'm also not sure if the Catholics (in general) would necessarily be all pro-Islam/Hinduism/Buddhism either. A lot also depends on where these immigrants settle. Many, of course, in New York, or New England, or for the Asians, California or elsewhere on the West Coast, but probably there'd be many scattered communities elsewhere, including in places you'd least expect it (like OTL Tarpon Springs, Florida and their massive Greek population).