To start with, I suspect they avoid this as much as possible. In the preliminary rounds they'd probably pair up elemental v elemental or pilot v pilot, as much as they can. Even a mechwarrior opponent for either would make more sense.
Sadly, in Phelan's known example, this doesn't seem to be the case.
He ended up facing only 2 MW along with 2 Elementals, & 1 ASP.
That would be a LOT of other phenotypes coming after that Bloodname if they had been paired w/ like previously.
But lets step back & think about this a minute from the Protocol & Decorum perspective.
1. 25 Heritages & based on the Alder Mathus example, they tend to get "typecast/profiled" into following a pattern where each Heritage is funneled into one of the phenotypes.
2. The 32
Nominees come from other Bloodnamed (24-25), the House Master (6-7), & the Grand Melee (1) (The variation comes down to if the deceased left a "will" with their nominee like Cyrilla did or if the House Master chooses it by default.
3. Most of the house is going to follow "norms" & nominate someone from the phenotype of that heritage. (Clanners don't like "Rebellion"/Wolverines, don't break faith)
4. For a Nominee to get pushed forward towards a non-matching phenotype heritage the un-blooded warrior has got to be pretty good friends with one of the Bloodnamed.
Not just "Your my Designated MW" kind of Friends, but, "I will go against the clan norms & nominate you for something that I normally wouldn't do because it will make me look bad" kind of Friend.
IE. Not just a Drinking Buddy but a I'll bury the Body w/ You kind of Buddy.
That, or the Un-blood is owed a huge favor by that Bloodnamed, etc etc.
The Cyrilla Ward example was held up as a "this is NOT the norm" kind of thing.
Her heritage was apparently one of the most valuable in the clans.
5. Finally, and this is just a personal perspective, but, any phenotype going for a non-matching heritage is likely going to be a target from everyone who is "supposed" to be there. The danger is, if you win the flip, then you likely have an easier time but the animosity is going to be huge. Meanwhile if you Lose the flip, then the standard phenotypes are going to be looking to KILL you, not just beat you.
Phelan was being very kind and un-bloodthirsty in his Trials, but it was noted that Vlad was not & was killing his opponents IIRC.
Augmented Elementals won't get wounded & signal surrender, they will get Stepped on or Alpha Striked till there is nothing left.
Un-Augmented Pilots will likely get their necks snapped, etc etc.