There are still quite a few races not accounted for in the Combine universe (you can get a feel for some of them by visiting the Race Suggestion forum). Many of those are near-human, and don't offer enough variation from current near-human races to be included.
Others haven't got enough info or graphics (as in the current Stenax race suggestion) to make them viable as player-races.
But as far as simply RP-ing a mixed-race character? i'd say go for it, as long as you allow your DM to have final say-so over the other race's characteristics, and how they are used within that RP.
Fr example, before the Diathim were approved, I doubt there'd have been many DMs who would have allowed a PC to go flying through space without any gear or hindrances. But now that they are an accepted race, there are many people who RP them for their rather unique skills, one of which is their flying capability.
I'd love to hear more about which race you're referring to, and the story behind the hook up between your parents.
It's hilarious and incredibly lame when people RP being a naturally born hybrid of something like a mammal race and a reptile race, for instance. The whole idea makes me want to go on an IC crusade and murder all hybrids. :p
However, the pseudoscience explanations are pretty common and can't technically be disproven with ambiguous SW technology, such as characters who are the results of lab experiments.
With races that aren't actually part of SWC (yet?), you shouldn't RP any unique characteristics of them and should really stick to the racial traits of the race that does exist in SWC.
RP is based on what the database says, and if you are making claims that contradict the contents of the database, they should avoid being seriously gameplay-affecting in nature. Different hair or skin colour doesn't affect much, but RPing as being able to turn invisible, communicate telepathically, or regrow limbs or something would be poor form.
Lame, as Syn said, but not worth arguing about if it doesn't affect gameplay balance. I don't think it's a White Scenario (or ScenarioHall)-specific matter though. 'All you have to do is find an agreeable GM and play your Hybrid Anzati-Falleen with both races' abilities' ? Nah, that's not reasonable.
In most previous cases I've seen of Hybrid-claiming or Re-Skinning, it wasn't too serious. IMO, best practice is to focus on playing the racial characteristics & abilities & limitations of the database race, excluding the abilities, & minimizing the major characteristics of the other race if you must play a hybrid.