Would I be able to mount a 503 head on this tripod

http://www.bhphotovideo.com/bnh/controller/home?O=3910&A=details&Q=&sku=272474&is=REG&addedTroughType=categoryNavigation

Or do i need a tripod head with a 100mm bowl base?

You need the 100mm bowl for it. I use these legs and can switch between my 516 head and my 503 heads with about 2 minutes of work.
Don

Im thinking about getting the 516 head, So I would just buy a 100mm bowl with the 3/8 screw on it and Im all set?

515 legs, 100mm bowl for legs, whatever head you want.
Again, my 515 legs with a 100mm bowl work with both my 516 and 503 heads.
Don