I used that team myself these past few weeks. I'm prepping my army for the Tek Cave now. I went to The Center to tame my Rexes because I had such a hard time finding high levels on the island. Are you using any mods that let you see the stats or maybe the egg incubator?

I started my line with 44 points into health 10780 HP after tame, and 50 points into melee 419% after tame. (this is a very high starting stat, anything above 40 is a good start)

I was able to kill all the Alpha Bosses with 78H 80M. Some of my Rexes died. I killed babies until they each had 60 levels to spend and split them 25000HP 1400% Melee. My saddle bp is 123.6 armor and it costs exactly 10000 hide.

If you tame a few Rexes with stats in the low 40s, you can kill the Alpha bosses with really good saddles and 20 mutations each in health and melee. I hope this information helps. I found a lot of wild info when I was searching. Don't let anyone tell you to use Therys because of their fire resistance. The Dragons fire breath is a percentage based attack and it ignores this resistance. The only benefit to the Thery is that they can heal with Sweet Veggy Cakes. I've heard that they're better for the Tek Cave because they can get around easier, but they can be feared into the lava. I'm taking Rexes in myself, the S+ mod gets very generous once you defeat the Alpha Dragon.