It seems like you are talking about
Line them up in order,
Alphabetical order (listing the alphabet)
And into teams as well.
Letters in teams could be considered words.
Each of them’s a character,
Characters (printed symbols) include letters.
Under someone’s spell.
Letters are used to spell words
Some have foreign accents
And put on different things:
A cute hat, something grave,
Tails and dots and rings.
These refer to the different diacritics added to letters in different languages, e.g, grave and acute accents, umlaut, cedilla.
The leader might be biggest,
Letters at the beginning of a sentence (or a proper noun) are capitalized and therefore, the biggest (credit to samm82 in the comments).
Original idea: Alpha (first Greek letter).
Or they’re equal in one case.
In upper case, letters are roughly similar size
Some will often go alone.
And neighbour only space.
Letters such as 'a' and 'I' can represent words on their own.
Sometimes leaders represent
Their teams in one location:
I think this refers to acronyms (leading letters represent words).
An initial strategy
For quick communication.
If you consider textspeak with things like "lol" and "omg", often acronyms are used for quick communication (note the play on words with initial too)