1) The bar spacing of a Ferret Nation cage is around 1in, which may be too wide for young/small rats. You could mesh the sides, which would involve using wire cutters to trim a roll of hardware mesh and attaching sheets to the bars/door using zipties, or you could get the Critter Nation model instead which already has the recommended 0.5in bar spacing. 
Personally, I wouldn't attempt meshing the sides unless you are getting a really good deal on the Ferret Nation and you feel comfortable working with hardware mesh. It's kind of a nightmare to cut, and can hurt your hands if you don't wear sturdy gloves. You may also have issues with the zipties if your rats get a hold of them from inside the cage..
2) Yep. It functions as a spot for them to mark / wee on. I'm not sure if we know 100% why rats do this, maybe it's that "pee rocks" stand out from the rest of the litter in the litter pan, but rats are typically drawn to mark them which can help a lot in litter training. 
3) If you already have a small bin cage that is portable and properly ventilated (with strong mesh & not plastic screen or holes in the plastic), go for it. But definitely don't make a bin cage specifically for this purpose. If you're considering making a bin just to use as a travel carrier.. I would go for something like this instead.
4) Diet is always super complicated. 
So rats are omnivores that really benefit from variety. But there aren't really any safe/suitable commercial seed mixes readily available in the US, unless you decide to trust an individual or rescue's homemade mix, so it's often recommended to make up the majority of your rats' diet with a high quality pellet food such as Oxbow Essentials Regal Rats.
You can absolutely supplement with a mix of safe/healthy seeds, grains (including pasta), dried veggies and fruits, dried legumes, etc.. And regular servings of safe/healthy fresh foods are also recommended. You'll need to do a lot of research yourself and end up deciding for yourself how to go about this- there isn't necessarily one right or wrong answer you can get.
As for portions, there isn't a one-size-fits-all answer to that question.. What I would do is make sure I'm using a high quality feed, limit the treats I provide to a reasonable amount, regularly weigh the rats to keep an eye on weight gain/loss, and pay attention to how much food is leftover around the cage when it's being cleaned.
Generally speaking, so long as you're keeping an eye on the rats' behavior and weight/mobility, you should be fine just eyeballing your daily portions. When it's just two rats and you're around to supervise them; it's better to overfeed by a bit and learn to correct your mistake, instead of agonizing over finding a specific number and ending up underfeeding.
5) Some folks just supplement their adult rat food with protein rich foods, and some specifically buy Oxbow Essentials' Young Rat formula or Halarn Teklad's 18% protein formula. It looks like the flavor of Oxbow Essentials' formula has improved so that more folks' rats will eat it.. So that might be worth a try. It's what I would do, anyway.
6) Definitely join a pet rat forum, such as this one. That way you'll have a larger base of folks to ask for support; advice; etc. You're not going to be able to get everything perfect on the first try, that's 100% to be expected and OK, so it's good to have a larger base of experienced folks to answer your questions and refer you to more info that can help. 
Also, budget! You don't want to be caught off guard by an unexpected expense. Make sure you have more than enough funds for vet care, supplies, etc. And make sure- especially if you're a dependent or don't have many transport options- that you aren't going to be trapped into not being able to access your chosen vet or refill food/bedding/other supplies. 
And definitely prepare yourself for a bumpy, stressful road when you first get your rats. You'll probably immediately love them because rats are amazing, but they're also quite likely to be skittish and shy at first; you may even get nipped or squeaked at; and there are most likely going to be some individual quirks/traits of theirs that you're not going to be thrilled about.
Don't expect everything to go perfectly, don't put other peoples' rats / rat experiences on a pedestal, and just make sure you're prepared to deal with the issues that may come up (whether that means dealing with something you already had in mind or asking questions about something you hadn't expected). 
