Can the length and first character or characters be changed in an altcoin address?
(E.g addresses such as 123345 could be generated)
Altcoins could make whatever they wanted
If they want a Base58 address, then they can change the first byte, but no more, unless it's a vanity address.
First byte = version byte. First byte doesn't necessarily equate to first character - depends on the first byte they choose - it's the version byte.
See:
https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses