The nest box should be added about day 28 from breeding and they will give birth around day 30-31. If you offer it much sooner they will use it as a litterbox and some does can't even have a box that soon. If you see nesting behavior though offer a box sooner.
I find goat kid formula mixed to double concentration is much better than kitten milk replacer. A rabbit is not a cat or even a carnivore and kitten milk is far from rabbit milk. There are a few other recipes online with some using regular goat milk but you have to increase the concentration or energy in most milk or forumula because rabbits produce very very concentrated milk. They only feed once or twice a day so the milk has to be packed with energy and they have to eat a lot of it until the kits look like ping pong balls with legs.
As to breed differences the small size of the netherlands requires a lot less food (they'll go a week on what my mini rex eat in about 2 days) and really can't do any damage scratching or nipping if they get wiggly. It's almost amusing when my smaller netherland buck gets in a bad mood and tries to tell me off. He can't even break the skin and he can't get through my blanket when he paws at it. These incidents are rare since he's generally a very laid back rabbit who watches movies with us. On the other hand I had a nasty mini rex doe tear through leather gloves and cause a lot of damage to that same blanket requiring sewing up several areas. Not that most mini rex are that bad. Amako is my little pet doe who has never done me any harm even on accident despite having your typical doe attitude where she thumps and grunts when her space is invaded. Mocha and his offspring were also very laid back and nice rabbits. I've sold several that were being bought for the children to handle and sometimes breed. You just want to ask a few more questions about the temperament of the mini rex you are buying.
Netherlands are slightly harder to breed. The litters tend to be small, some does just can't produce live litters, and you have a higher chance of peanuts which are kits that got too many dwarf genes and cannot survive. We haven't had good luck breeding netherlands. My blue doe is on her 3rd attempt after producing 2 completely stillborn litters. A himi doe I got never took despite breeding to several bucks. Shimo will probably produce better but she does not have good netherland type and size which is an advantage when trying to get good litters but those litters will not be show quality. The mini rex have a few misses and sometimes peanuts (I've only had 1 doe and buck pair produce peanuts) but overall they breed pretty easily and most litters survive. Some do have small litters of only about 3 but some lines will produce litters of 5 or even 7.
On the show end mini rex have a lot of competition and you really need to know how to pick out something of quality to get anywhere if you are breeding for that purpose. Some colors are also a bit more difficult to make show quality. Some colors should not be bred to other colors unless you know what you are doing because it will alter the other color. I don't have as much experience looking at show quality netherlands but type issues seem to be more of a problem than color. Getting that upright body, small size, small ears, and round head can be difficult. Especially the round head it seems. I haven't found a doe I wanted in my price range that had a really round head like my 2 bucks. I'm not breeding for show though so I don't care too much. In both breeds and probably most breeds the self colors of black, blue, chocolate, and lilac (chocolate and lilac is not always an accepted color) are some of the easiest. They produce few color oddities and usually only the occasional toenail issue. They are a bit boring compared to wide range of colors you can produce though so it can be a difficult decision on what colors to attempt. Personally I love broken chocolate and sable but brokens have their own set of breeding problems and sable requires using a himi line because sable to sable results in some darker rabbits called seal.
