It is not best for the puppies to be altered before they leave you. Unfortunately it is the only way to ensure that the puppies are not bred. Just because these are rescued puppies does not mean that it isn't still better for them to be mature prior to altering them.
It is far cheaper to alter puppies as the amount of anesthetic needed is dependent on weight and so the price will go down significantly to alter babies. However, the hormones that you are removing do make a difference to growth and many people think longevity as well.
Altering these puppies yourself will still run you into costs that, frankly, you don't deserve, and it is very likely that you will not recoup those costs for what you can charge for the puppies. On the other hand, you really don't want anyone to go through what you went through with this girl.
I think whelping and raising this litter has been more than what anyone would be expected to do, and you are doing great with that. Having a good rescue help place the puppies may help find the best homes for the puppies, though if you do have people showing an interest in the puppies, you might decide on whether or not they will be good homes yourself.
As for the bitches, there is no chance at all at these two being compatible, while the one has puppies. So no, do not even try that. There is no point at all in trying to socialize the puppies to this other female. It is far better that the puppies have positive experiences, and you do not want the other bitch's smell on the puppies. Remember that dogs can smell a drop of urine in something like 1000 drops of water.
Good socialization experiences during this period are very, very good. Bad socialization experiences are very, very bad. If the other female were to frighten or hurt even inadvertently one or more of the puppies, that can be worse than simply allowing the bitch and pups enjoy each other's company. When the pups are placed, there is plenty of time for the puppies to get 1 on 1 socialization with other dogs.
Lastly, there is no way to ensure that your females will be buddies after all is said and done. Spaying can actually make things worse, or at least spaying can increase reactivity in dogs. On the other hand, spaying can remove the hormones that cause the bitch to go in heat and shakes up her system physically and behaviorally. So it is possible that it might make things different. Generally people encourage neutering for behavior issues in dogs, but do not encourage spaying for behavior reasons.
I think that maybe your best bet would be to find Heidi an awesome home and keep a male pup out of the litter. But if this whole experience has made you completely attached to the girl, crate/rotate, upstairs/downstairs might be in your future.
I think you need to think and discuss this with your husband, with the ideas of what is best for you and him, what is best for Sacha, and what is best for Heidi. Sometimes bitch-bitch does not work out, and even with a system in place to keep them separated, shtuff happens and the results might be serious vet/medical costs and even death of one or both of your furry charges.
I am sorry you are going through all of this.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC