I don't know a thing about mlp and I don't plan on watching it. The 2 shows are very different and one consists on animal characters only, so I do not think it would be "epic" just strange. I'm expressing my opinion so yup.
If Hetalia and My Little gppony, pony combined, Russia would scare the living hell out everyone (as usual) Italy would run around trying to make friends, for England it will be normal, America will be all "DUDE THIS IS TOTALLY COOL!!" Japan, China, and Germany will all be deathly confused, and France will try to do unspeakable things. >.>
Best friends: Italy and Pinky Pie Japan and Fluttershy America and apple Jack Germany and Twilight(Maybe...?) Hungary and Rarity Belarus and upinde wa mvua Dash(I could see that happening.) Romano and Spike(Pfft, I dunno)
Worst enemies: Germany and Pinky Pie(Although they might have another Germany and Italy type relationship...) Switzerland and Rarity(Man up, little pony, au I'll beat wewe with my peace award!) Japan and apple Jack(He's zaidi likely to be afraid of her...) Russia and Fluttershy(Russia: Kolkolkol... Fluttershy: ...Eep!!!) Belarus and Fluttershy(One again, Fluttershy is bound to be scared of her)