If you have fish with an aggressive nature, like the texas, size is not the first thing that will trigger the aggressive attitude.
Both fish will grow, and even though it may take a while, aggression issues may come up eventually.
The only way to be sure nothing bad happens is not to combine the fish.
The size difference will not be any guarantee on the long run.
I had a Vieja in with my oscar that was smaller then the Oscar and there was no aggression in the first few months, but gradually the Vieja started to feel at home, and took over more and more of the Oscars territory.
I tried to correct any negative behaviour of the vieja by threatening to net the Vieja any time she started to bully my Oscar and it seemed to work for a long time, but things turned worst when i was not around.
My oscar was bullied more and more and took a beating from time to time.
But in the end, one night the vieja had a go at my oscar and had my o knock himself out completely, and left him battered and bruised ,floating unconcious for quite a while.
I rehomed the Vieja the next day...
you can try the combination, but better make sure you keep a close look on their behaviour, for the fishes attitude towards eachother can change within a short period.
A lot depends on the attitude of the Oscar to, for my Oscar is a very gentle and laidback fish to begin with.
plus if you decide to try the combo, make sure you got a backup plan, like i did, so you can rehome quickly, if it is nescesary.
All this goes for aggression in general, for i have no expierience with the combo you suggested, so maybe some other members will have other advice to add to mine.
If i'm right,Cris or longhairedhippyweirdofreak has expierience with the Texas and other aggressive fish, and may have more to add.
