In rugby the only position restrictions is the law that only players trained for front row play are allowed to be in the front row for contested scrums. Other than that pretty much anything goes. At the elite level, flankers are often used as jumpers in order to provide more lineout options. But there's no rule saying you can't throw a centre or two into the lineout if they are particularly good jumpers, but it would also be wasting their potential speed once the ball comes out. About the only real limitation you will see is that your props will be on either end of the line, but that is for purely practical reasons.
No, you will often see the number 8 and blindside breakaway jumping. There is no restriction on who can jump (or who can throw into the lineout).
