Since rexs are the second best land carnivore (behind giga) I would recommend just leveling it for PvP. Like on all my 150 kibble tame rexs I go for 10k health then 500 melee damage and then put the rest of my levels back and forth between health and damage.
PS: A good 2nd-5th gen Rex can and will kill tamed gigas
When breeding two max level rexes to have a super baby, DON'T EAT THE EGG!!!! I've done this twice on accident(meaning to drop it) and each time it takes me a couple seconds to register wtf I just did and then I have to wait to mate again. Sad day. Weekend ruined. Great way to start off the new year! Lol
Rex baby consumes meat at about 1 meat per 50 seconds. Each meat gives about 50 food, with a food drain of 1 food point per second. So.. 1 meat = 50 seconds of feed. 1 stack of meat is 1000 seconds/16 min. A stack of meat takes 12hrs to full spoil. So.. on 1x, at 6% maturity, a baby rex can be left to fend its own till juvi... this is even with a 2 hr safeguard to be very safe. Honestly... 5% would work also. Hope these statistics help!
From my experience my rexes have layed a bunch of eggs I have 2 females and 2 males and I was able to get 120 eggs in the past 2 weeks however you may not have the same luck, however taming 3-4 females and a male should enough to supply you with plenty of red eggs to tame quetzals and plesiosaurs
Good for getting prime because they only will get attacked by the allosaurus Rex and gigas recommend getting a low level first and then moving on to tame a higher level later on rather than taming other dinos for collecting prime