Yes, don't let him scare you off!
There's nothing wrong with going oval.
To answer some of the scares he threw your way:
Walls only heave if the ground at the walls is completely saturated and freezes - therefore, a well drained trench with lots of 3/4 crusehed gravel within a few feet of teh pool wall is imperative to keep the moisture content at a minimum - especially if there is an uphill section off any part of the pool which would cause water to flow towards the pool.
I highly doubt anything UNDER the pool will ever heave - once the pool is installed, WAter really can't get under it, so unless you have an extremely high water table - the ground under the pool cannot heave.
As for the bottom rail corroding - why would that only be a problem on an Oval???
Just make sure you support everything properly, compact really well, use sand or even 2" blue styrofoam as a base under the liner.
Also, crusher run is a great product to use as a sub surface for the bottom rail - it helps with drainage and becomes very hard once compacted - make it about 2-3" thick compacted - it will also allow you to get your pool perfectly level - a nice product to use.