I'll start off by saying that I'm no expert, but I've had my brb for a year now and I stalk all brb info I can.
I'll try to remember all the unanswered questions, feel free to re-ask one of it was missed.
Should you feed pinkie or fuzzy? I would go ahead and do the pinkie for the first feed. You have it already and it's gentler on his stomach right now after stress and shipping. Many breeders start off with fuzzies, but feeding the pink will be fine.
Try not to handle so much. I started handling after a week and in the second week, I almost got bit in the face. My brb is the sweetest snake ever. The only thing he hates is being released and then 'recaptured' and being measured [they go hand in hand for me and this is very common in brbs]. You can sit there and boop his nose three times before he moves his head, that's how calm I'm talking here. But he was obviously still somewhat stressed and didn't know me and was hungry so he went for my face [faces are scary for little guys]. I know it's incredibly hard, but your snake will be happier if you don't touch it at all for a solid week. I peeked on him daily to make sure he was alive but then left him alone.
When should you move up in enclosure size? This is debatable and only you can answer it but we can help. The general rule is that an enclosure is too small if your snake takes up two sides of the tank when stretched along the perimeter. In other words, while the snake is crawling around, if it stretches along all the long side AND the entire short side, the tank is too small. For larger snakes you can do the snake length vs feet thing too. You need at least 1 square ft per foot of snake [3ft snake in a 3x1ft enclosure for example.
Cage suggestions? I absolutely have them! Firstly, you're probably going to want something between the 10g and the adult cage. I HIGHLY suggest just getting a plastic storage bin. They are cheap [you'll only have the cage for maybe a year?], customizable, secure, and GREAT for humidity. The clear ones aren't as clear as glass but you rarely see nocturnal snakes like brbs anyway.
This is mine, decorated for Halloween.
It comes out to about a 3x1 cage and I've had him in it since he was 2 months but I absolutely littered it with hides. I have a heat pad underneath the bin attached to a thermostat. I use reptibark topped with moss and it holds the humidity stupidly well. I live in a desert where it gets into the single digits for humidity and with the house a/c and the set up, I only have to spray about once a week to keep the humidity at 99%. [I either get 99% or rapidly dropping to 75-80% or lower, no inbetweens]. The bin cost me $15-20 and I drilled a few holes in the sides. As you can see, it's very clear. It also has large locks on all four sides and is extremely secure. Rainbow boas are very strong escape artists.
Adult cage
Or you could go straight for the adult cage. I highly suggest AnimalPlastics. I've heard mostly good things about them. People rarely have truly bad things to say. Their customer service is nothing but fantastic and I have never ever heard a complaint about that. You can get a 4x2x15" cage shipped to your house for less than $300. It's made out of plastic so it's light enough for one person to manage [though bulky so I suggest help if you can get it] and very mold resistant where melamine and plywood have to be EXTREMELY carefully sealed to prevent any wood from completely destroying your cage. The AP cage should last the 20+ years, at least most of that, and is easy to customize. You can also ask for less ventilation which will help a ton with humidity and they are happy to do that for you.
Boaphiles are another popular brand. Very similar to AP cages however you don't have to put them together and seal the joints like you do with the AP cage. However, Boaphile is pricier.
Stay away from getting anymore tanks, it's not something I see a lot of people sticking with when they have brbs because it's somewhat of a hassle. Like I said, melamine and plywood [especially] can have mold issues if not sealed extremely well and completely.
If you build a melamine enclosure, you'll have to price how cheap you can get the melamine, plus proper sealant, hardware like screws and such, and the hardware and acrylic or glass that you use for the doors. It might be cheaper depending on what prices you can get. I don't know enough about that to be able to answer it. If you can find cheap stuff, you might be able to but in my experience those types of projects tend to take more money than expected. I might just be poor at planning project budgets haha.
As for cages, more room is better. They'll use the height if you give it to them. I would do a 4x2 floor space minimum which would cover a 6ft snake. The height depends on what you get or what you build. More is better. They aren't arboreal as adults but I've heard that they'll still use it.
I just wrote a book, I think I'll stop now.
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