I guess the DNA evidence answers part of your question.
Is it true that Australian aborigines set foot in the Americas before the mongoloids and did they mingle with them scientists claimed to find a 56 thousand year old aboriginal skull as well as aborigine dna in some tribes?
Unlikely. The Aborigines arrived in Australia via a land bridge from Asia (apparently via what is now Indonesia). They have been here for at least 40,000 years.
Its possible they shared a common ancestor with those who settled in the Americas, but it would have been a pretty good walkabout.