Each cryptocurrency address has a specific leading character, ex. 1 for Bitcoin. Does that mean that there can only ever be 54 alternative cryptocurrencies created?
I based this on the fact that O, 0, I, l cannot be used in an address. Because of that there are 46 possible unique letters and 9 unique numbers that can be used as the leading character in an address.
Can 2 coins have the same leading character and coexist?
Yes, the leading character doesn't matter, they are on separate networks.
The leading character does matter if that is the only way to distinguish between addresses of different types of coins. There was a post not too long ago by someone worried that he lost some coins accidentally sent to a TRC address through the BTC network (The coins were recovered).
It should be noted that the '1' is not simply prepended to the address. Instead it is the result of encoding the version byte value, which is 0 for normal bitcoin addresses. See
https://en.bitcoin.it/wiki/Base58Check_encoding.