Hi Pixie and welcome to the forum
In terms of specific advice, with Rex breeds, they are prone to getting sore hocks (sores on the bottom of their feet). So a variety of soft and padded flooring surfaces are ideal to ensure this is prevented. They don’t have the thick fur covering the hocks that other breeds do.
If you make any dietary changes, do so very slowly. Rabbits have very sensitive digestive systems and any changes should be done gradually. That includes changing between pellet brands and adding in new foods such as leafy greens. Unlimited grass hay, 1/4 cup of plain quality pellet (no seeds or grains or muesli mixes) until 6 months then you can reduce the amount, and from 3 months you can try to add in one leafy green at a time.
Bringing home a litter mate can be a challenge, while having a pair of bonded bunnies is fantastic, the process of bonding can be quite difficult and litter mates do not always get along past puberty. It is recommended that both bunnies to be bonded are already spayed/neutered before attempting bonding, so getting two young rabbits would only require separate housing until they were neutered and could be bonded properly. Rabbits are sweet creatures, but they can be extremely territorial and violent towards other rabbits in their territory.
Feel free to start new topics on any specific questions you may have 