interbreeding

Question
I rescued some feeder mice that the feed store clerks sexed wrong. (I asked for several females).  After all the babies were born, I separated them into like-genders.  Problem is that after awhile, the males started damaging each other quite badly.  I eventually separated them.  The girls in groups are fine, but the little boys are now all single and alone. (I have eight loners).
I've tried mixing and matching the different boys according to (what I perceive) are compatible personalities, but they always fight to the point of blood and raw skin at the base of their tails.  I cannot play with them individually enough to compensate.
I have also rescued field mice from various dangers on my farm.  I've kept a few, and had great success with them.  They have lived long and been healthy.  Once, I put a male feeder mouse and a female field mouse together.  They lived together for about two years until she died of natural causes.  SHE NEVER GOT PREGNANT.
My questions are:  What are the chances of reproduction between male feeder mice and female field mice?  Are they any different than for mice who are the same? I have some female field mice I could place with my lone male breeder mice, but I am not anxious for any more litters. Since my one experience with this was positive (no babies), I am wondering if I should repeat it.  I hate for the boy mice to be alone.  (My vet would neuter them, but he says there is a high risk re: anethesia and it is expensive.)  If you acutally read all this and paid attention to what I wrote, thank you very much.  

Answer
I have never heard of a case between a field mouse and house mouse successfully mating. If that ever did happen it would surely be a first! As far as the male mice as concerned, when male mice reach adulthood it is best to house them seperately. If housed together they will constantly fight for dominance, which is putting them in harms way. Male mice can and will fight to the death. It does not take much for their tiny teeth to cause serious damage to another.

As far as trying to house your males with female field mice, the most important thing to think about is exposing your house mice to potentially deadly viruses and bacteria, as well as parasites that the wild mice may carry. Another concern with keeping wild mice is the chance of contracting the hanta virus. If you do decide to house the two together, keep an eye on them very closely for the first week. It would be a good idea to keep a log on behaviour, social interaction and eating. If after two weeks everything seems fine I don't see why it would harmful keeping them together. Just monitor for any signs of illness.