What colour would normally come out of a black dad and palomino mum? Offspring Color Probability
16.67% - Smoky Black
16.67% - Buckskin
16.67% - Black
16.67% - Bay
16.67% - Palomino
16.67% - Chestnut
This doesn't take into account the probability that you could get the Appy pattern and not knowing some things, like whether your mare carries the silver dilution gene, or whether she carries the agouti pattern, things like that I can't give you any closer probabilities for color that what I've got.
Would you allow it to happen?
I would not. The reasons I wouldn't are because there is not much market for horses right now and an unregistered one (mutt) is harder to sell than a registered foal. You're talking 2 crosses which unless you can go with a color registry are not going to produce a registerable foal with any breed registry. Is either of the proposed parents registered? If your mother's mare is registered as a half - Arabian, she can be bred back to a pure Arabian stallion for a registerable 1/2 Arabian foal.
Now if all you want is a foal that you can play with and possibly keep for its lifetime, that's a whole different consideration. I breed only for registerable, salable horses and with the economy downturn, I am breeding less and less foals each year.
If you want to allow the breeding and give the farrier his foal, I'd do it for a certain number of shoe-ings or trims and let it go at that, I would not do a foal/foal swap.
How long does the foal need to be with it's mother/feeding?
My ideal time is 6 months but will pull a foal at 4 months if it's huge and really draining the mother. You have to increase mom's feed (pretty much double or more her grain) to support the lactation and feeding the foal so it gets pretty pricey. At 4 months if the foal is eating well and just emotionally dependent on mom then I'll wean them and let them stay in a neighboring stall or paddock for emotional support until they are 6 months or older. I, personally, like to leave the mare open for the year after she has foaled, it gives her time to raise the baby and for her body to recover from the stress of the last 18 months.
How old do you think is too old for a mare to become a mum? I'm guessing you're really asking, for the first time? I wouldn't breed a maiden mare past 15 years old unless she was really healthy and sturdy and the vet agreed. If she's not maiden, just not been bred for a few years, then it's really up to what the breeding soundness exam shows. Again, your vet can probably steer you best here.
Would the breeds make a nice looking horse?
IMO, it's not a cross I'd ever be interested in.
What is the chances of it going wrong?
Arabian mares (and as a 1/2 your mare probably got all the good breeding karma) tend to be the easiest to get and keep in foal. They also have a lot fewer foaling problems than say warmbloods. I never realized how easy us Arabian breeders had it until I was talking to a friend who breeds Warmbloods. My goodness, the stuff they seem to experience yearly I haven't seen in 20 years. So, while there is always a chance something will go wrong, with the Arabian blood you have in there, I'd say your chances of everything going right are a lot better.