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.