Well, I agree with Beerson that introducing Homebrew is difficult. It might be accepted in a narative campain where one person acts more like a RPG-like Game-Master than a player, but otherwise other player (and yourself) will always wonder if those are fair (or too weak).
Luckily, people like us ( = people who like converting and messing around with units) can be verry happy with the current edition:
1. The most simple Game-Type offered in the Rulebook even offers to play with "all miniatures you own" (not a direct quote, since my rulebook is not the english one).
2. The indices have a pretty nice wording in most cases: "A ... is a model armed with ..." (direct quote this time). Now THAT is nice
. Using a Warhammer-Fantasy-model as, let's say, a Tyranid Lictor is allowed even RAW. (...and other players will in most cases be ok with it anyway, as long as you use your models to enhance flair and not just as a placeholder to get powerful units you do not "own".)
3. Many opponents will want to play Battleforged Armies. Combining different Factions to one Army is not allowed in a Balanced Game (though it is in a Free Game) (I hope I get the English translations right...).
But in a friendly environment nobody will object if you play one Detachment of Tau and one Detatchment of Seraphon that use the Tyranid/Ork/Something-Rules.
Most important is, of course, to choose the rules/unit that seem best to describe the model, not the most powerful.
...Though: If your goal (and source of fun) is not in playing/modeling/narating cool auxiliaries, but in creating new rules... well, in that case you can forget all I wrote. In that case: enjoy writing those rules :-)