The only rule I was ever given is that an ett word that ends with a consonant uses the same word for the plural. Example, ett hus, två hus etc.
Other than that it seems to “if in doubt use en as 75% of the words are en words”.
Being a native speaker I’ve never had anyone try to give me a list of rules, but I have been asked about it which set me to thinking, and I did manage to come up with a few rules.
[ul]
[li]Words ending in -a are almost always n-words (exceptions: komma, tema, schema, A (the letter)).[/li][list][li]Words ending in -a with a plural on -or are always n-words.[/ul][/li][li]Words ending in -are are always n-words (these are derivations of verbs, like the English suffix -er, as in write -> writer)[/li][li]Most words meaning some kind of human are n-words (exceptions: barn, tidningsbud, a few others).[/li][li]The reverse of the one you mentioned: words whose singular and plural are the same are almost always t-words (I seem to recall finding an exception to this, but I can’t think of one now). I didn’t think of the connection to ending on a consonant, though :).[/li][li]All letters of the alphabet are t-words; all numerals are n-words (all numerals are -a/-or words, actually).[/li][li]Obscure and probably pointless: Words ending in -nde are present participles. A present participle can be used as a noun in two different ways: as an n-word it means “someone who [verb]s”, as a t-word it means “the act of [verb]ing” or “an occasion of [verb]ing”.[/li][/list]
Recent loan words also seem to end up as n-words, though this may be mostly a question of most words being n-words, as you noted.