Logo sizing and editing

I have a logo produced on Looka. I have downloaded various files (png, svg, eps, pdf, jpg) , (colour, black, white), (with background, without background), (web, print, social media) with all combinations of the attributes in brackets. The logo consists of text and a skull. The files referenced above are all different sizes in terms of dimensions and KBs. Because Looka only allows 1 x icon ( in this case the skull) in a logo and I wanted the design with a dart in the skull I have to add the dart ( an adobe stock vector image) in Illustrator. Please can someone advise how I can size and position the dart in exactly the same place and dimensions relative to the varying skull size in each image . 
Many thanks for any help