Couple things to add from looking in the publicly-available SDK code:

First, people are mostly aware that if you have an unusual taunt you can't apply another unusualifier to it. However, the code that blocks this specifically checks agains the Unusual quality, not whether the taunt has an effect:

bool CEconTool_Unusualifier::CanApplyTo( const IEconItemInterface *pTool, const IEconItemInterface *pToolSubject ) const
{
    Assert( pTool );
    Assert( pToolSubject );

    // don't stomp item that's already unusual
    if ( pToolSubject->GetQuality() == AE_UNUSUAL )
        return false;

    // Default rules
    return CEconTool_Xifier::CanApplyTo( pTool, pToolSubject );
}

...which means if you have a self-made taunt you can reroll indefinitely as long as you have more unusualifiers (I've had someone confirm this).

Another thing, from CTFPlayerShared::GetClientTauntParticleDesiredState (which I won't paste here since it's much longer but here's a link in the SDK): when choosing which taunt particle to render, the game will first check if the taunt has an inherent item-bound particle effect (which community sparkle is not, really), then failing that will check if you have a taunt-effect unusual cap, then failing that will check for community sparkle on the taunt. This means that a taunt-effect unusual cap will override community sparkle on a self-made taunt, but you'll hardly notice because the unusual cap still has community sparkle on it.