I don't know if you want to use the term 'breeder' to describe your ownership of rabbits. Even 'pet breeder' since there's enough folks out there dissing breeders and having a 'pet breeder' doesn't help. Basically when you breed shelter pet to shelter pet all you get is more shelter pets. Sounds harsh, I know, but (IMHO, Hmm, maybe IMNSHO) there's a different mindset to breeding than pet ownership.
If you're going to breed rabbits, what's your goal for breeding them? Breeding a 'pet' bunny isn't going to pay very many bills, it may not even cover the costs of producing them. If you were to breed pure bred rabbits, then you *might* be able to sell them at a profit. Not guaranteed, but much more likely. If you just want more rabbits for yourself, then it doesn't really matter.
At the moment, you've got a fairly young New Zealand buck but only one buck. So, all the offspring will be at least half siblings and half New Zealand. That's a large breed and usually raised for meat and usually offspring sell for between $15 to $30 each. Not a lot of profit there unless you're doing economy of scale and raising them by the multiple dozens if not hundreds. Holland lops usually sell for a lot more, so breeding Holland lops might show a profit. But if you have a NZ buck, you don't get Holland lop offspring. Not even sure what you'd get as a cross between those two breeds.
You've mentioned two unknown breed 'maybe Holland lop' does. I'd guess the older one is too old to breed, especially since she's been around the buck for seven months and no offspring. She may be significantly older than the 3 or 4 years they said she was, but not a breeding prospect in any case. The younger doe at seven weeks old shouldn't have been sold yet. I think federal law is eight weeks. But, that's a side issue. Also, don't most shelters spay their animals? At seven weeks, she is too young to be spayed, one would think. Maybe she came from somewhere other than the shelter?
A Holland lop is around 4 pounds when full grown. A New Zealand is around 9 pounds when full grown. IF you had a NZ female and a Holland male, then there would be less likelihood of birth complications. There's enough difference in size that you may lose the doe from being unable to birth a kit that's too large? I don't know, but it might be something to look into.
So, let's work this through. Your young maybe Holland lop doe gets old enough to breed, she meets the NZ buck and manages to have a litter without excessive complications. Six months later, they're adults (providing they survive, of course, newborn baby bunnies are extremely delicate). Half of those adults are male and the best genetic match among the available does would be the older doe, but she's not breeding. So they'd have to be bred back to the young doe who is their dam. That's extreme 'linebreeding' (I consider it inbreeding, but not all breeders think that way), but for one generation is sort of acceptable - if you have a good reason to do it. The offspring would be 3/4th Holland and 1/4th NZ, although if the young doe doesn't look purebred, the offspring will look even less Holland. The young females can be bred back to the buck for one generation, although that would make the offspring 3/4th NZ and only 1/4th Holland and Holland would be the ones that could be more pet market, one would think?
So, use the NZ buck once to get some offspring. Breed the most Holland like buck offspring back to the dam. Do that again and you'll eventually get more and more Holland like offspring, but they're gonna be terribly inbred and you'll start getting inferior stock as the recessives start matching up. You could reach the point of lethal recessives matching up. It really doesn't work to start a breeding program with only one pair. You also aren't going to get much more Holland like than the original doe and you'd mentioned she doesn't seem pure Holland.
If you're going to breed, getting good breeding stock of some sort of purebred type may be very helpful. To start with, replace the NZ buck with something Holland lop-ish. Since the buck breeds more than one doe, the quality of the buck is more important than the quality of the does. Go find a purebred and high quality Holland lop buck, preferably one with a pedigree, but if he doesn't have one, start one. Breeders are always much more likely to sell off a quality buck than a quality doe since half the offspring are males and males need to be kept in separate quarters. Two breeding males will fight with each other and can even kill one another. If you want males for pets, get them spayed, but that costs $$$ so most breeders are always trying to sell off bucks. Even if you get them without a pedigree, you'll still be way ahead of using a NZ buck.
The non-bred does can be kept in a communal space, but they should have a separate space when they're nesting. Baby bunnies are seriously delicate and with other adult rabbits around, the newborns can get stepped on and crushed.
Because of your very limited genetic pool, start a pedigree on the bunnies even if you don't know much about their background just yet. When you get several generations down the line, being able to know their relatives will be important. Try to keep the coefficient of inbreeding to less than 25% if you can. Kintracks is an inexpensive pedigree program that is free to try out and has a handy 'coefficient of inbreeding' button on it.