The giants might also look alright on the Draharan Elephants. I've never tried the elephants, but I always stick to the same size restrictions you mentioned for my dwarves. For this reason I wish there was a goat that required less than 6 riding skill. I always rename my mercenary dwarf "riders" to "squires" or something because they cannot ride a goat yet, so I make them stay on foot.And for Giants: cows and Sut chargers, also for size reasons.