"Hullo" and "Hallo" are simply trying to spell the word "Hello" the way some people say it causually. "Hullo there!" I would not write it that way. I suggest you stick to "Hi" and "Hello." "Hi" is informal.
Cheerio is probably the word you mean, and its use is British. I don't know if it's still used, and I thought it was a farewell, not a greeting, so I will let a British English speaker answer that.