Assuming you're talking about the Scout Rank, it's possible that you can only use one monster (if you're going with ???Rank monsters), but as far as I know, what determines your rank on WMC is the stats and skill points (allocated) on each of your monsters. So in theory, you could use high rank (S, X or ???) monsters, as long as they have low stats. Try placing one of them along with a healslime or something like that and see if your rank rises.

Also, statwise Wildcard is a little bit better than Ace of Spades, but not much (specially if you're not planning on maxing them, in order to play on lower ranks). So you could go with the one you think is cooler. =]