For a novice user, i would reccomend that the prop sit's lower than the hi-jackers themselves. If it is level with them, then once the boat hit's a planing angle, then won't be enough prop in the water, and you will experience slip issues. This is where a SS prop, even a cleaver style come into play. An SS prop will flex less than an alloy prop, meaning you can run less of the prop in the water.
With regards to your saleman's reply, thundercats work with alloy prop's if not intended for racing use. I know this from experience with both mini and full size cats.
I'd argue if a salesman is reccomending a cleaver for non racing, general use. Ring another store, and ask them for their opinion on a default 50 HP prop modified for a more aggresive cup.