Toms can be picky about who they breed with, but I think it depends on the hen and not the breed. For example, my Standard Bronze tom will breed with my Spanish Black hens and my lone Standard Bronze hen, but my Spanish Black tom will only touch the same Spanish Black hen (and he bullies my Standard Bronze hen!). So I believe it's a personality thing. Kind of like people, I suppose.
Heritage turkeys will mate and lay eggs all summer. Mine have just begun laying (they began on the 25th of March) and I am up to 13 eggs already (granted, this is between 4 hens) but they should continue until the days start getting short in October/November. At least that's how it was last year. I do collect the eggs every day and don't allow my girls to go broody. I don't know if this is the same for Wild Turkeys... Oh, and my turkeys will mate regardless of what time of year it is, they don't care, hahaha.
I wouldn't mix young turkeys and older turkeys together, unless the younger turks are large enough (if they're almost full grown). I would also allow them to see each other through some fencing for a while before you put them in the same pen (that way they can get used to each other and not see each other as an intruder, well, I guess that goes for any age turkey you're trying to introduce). Sometimes you can get a good idea of gender once they're around 3-4 months or so, at least that's how it's been for me.

A good indication of gender is: once the feathers have fallen out of their necks/heads, the girls should have a thick "racing stripe" of feathers along the backs of their necks, while the boys will have barely any feathers or no feathers at all. Then the boy's snoods will get just a liiiittle bit longer and generally shrink and expand more. The boys will get tiny bumps on their legs where their spurs will be when they're around 6-7 months, and of course, the beard, but I haven't seen a beard on any of my birds before they were 9 months old or so. Not to mention sometimes hens can have beards, so honestly, I don't even bother looking for a beard.
Or, if you have pictures and the turks are old enough, you can always post pics on here and ask, and people will try to guess! Lol!
I hope this helps!!