You use Ancient Song in battle and it changes form. So if you had both and one used AS, you would be breaking any currently proposed change of species clause I've seen mentioned in the thread. Melotta can start off as either form, although it seems the move is not available yet, according to Serebii.
Sword of Mystery is Kerudio is so boss. I think GF actually caught onto (and abused) the fact that fighting is the ultimate second type. Literally, you can tack fighting onto just about any other type (except ground) and get a better-than-monotype pokemon. That said, psychic attacks are surprisingly popular, and not just Psycho Shock (of course this could be a product of so many awesome fighting types). I've seen more than a few psychic-using Slowbros!
And I'm having the hardest time staying on top of BW. Recently I've been having some fun playing on PO, but there's so much new stuff going on it's kind of hard to follow. I can't tell you how many times I have earthquaked balloon pokemon, and wondered how Metagross flamethrowered my skarmory. lol
In the huge city you have go to the left most walkway (starting from the place you entered, so the entrance to the city is the right.) There you should see some team plasma and more talking. then you'll fight them in front of the gym then you'll be able to fight the gym leader I think.