New Horse

In my experience horses can escape but this is caused by human error, fright or poor fencing. There isn't really anything you can do except use common sense, regularly repair the fencing and have plenty of forage (grass/hay) to keep them busy. If he/she does manage to escape have a scoop/bucket of nuts to rattle and they usually come running. Then find out how they escaped and fix the problem, so they cant do it again. As for introduction, ideally in a perfect world you should section a part of the field off and put the new horse in there and let them make friends over the fence. Fighting and biting is normal horse behaviour as they are sorting out the hierarchy.
I think it is 1 acre per horse and half for a pony and as for gender preference i prefer a gelding as mares can be moody and tempremental.

you should probably get a gelding just because most of them are calmer than mares and i think for every horse you have you need 2 acers of land

Well Geldings are lots of work cause after the age of 8 they start geting joint problems and every month they will need a injection (shot) to keep their joints running . And also to interduce horses together you should first bring them together on their halters and if their good together you can put them in the meadow together but you have plenty of room . Dont worry .:)