Some free ai files have licenses that say they can be used only "as is", or non-commercial, or require link back, etc.
Check the posted requirements for use.
YES, The beauty of vector images is the abiltiy to use all or none of the components.
The right program can bring that image in, break it into pieces and allow you to modify, stretch, shrink, delete, add to it to get exactly the look you want.
I sometimes bring in ai files to illustrator, export it as PDF and import it into Xara so I can play with them in Xara.
I can use illustrator (somewhat) but MUCH prefer to work in Xara products!