What is the crystal range for a dsPIC30F5016 to get max MIPS?
The clock chain is pretty flexible.  There may be several different divider and multiplier settings that cause the same ultimate processor clock frequency.  However, each part of the chain has a valid frequency range.  You have to make sure these aren't exceeded.  Depending on the frequencies you are using, there may be divider and multiplier values you can't use because the frequency range of one or more sections would be exceeded.

If you want to see if a 15 MHz crystal would work, walk thru each of the blocks picking divider and multiplier values as you go along, making sure the frequency ranges are adhered to.  If you find a solution such that all frequencies are in the valid range, then you can use it.

One advantage of 7.37 MHz versus exactly 7.5 MHz is that the former is a "baud rate" value which makes UART communication over a standard baud rate easier.  This is often worth giving up the 1.7% from the maximum speed the processor could operate at.