no, the gtx 1060 3gbs is slower and has only 3 gbs of vram
there is no itx gpu cards
there is smaller and shorter cards but those models can work like that, all depends on the airflow on your case
if i was you i would get the rx570 or the 1060 6gbs, forget the 3gbs model completely
if you find a rx470 very cheap, is a valid option
you will find more gpus with 6 pin than with 8 pins, just do a research on the models you find and the requirements they have, if any of them mentions it needs more than 150 watts, that is a not for you, if they say 150 or less, go for it, there is at least 5 decent vendors with both 8 and 6 pin power connectors so there is lot to choose from online, on stores is another story