Welcome to the wonderful world of minis!
could I eventually keep a mini with the herd? or he-she should be kept in a separate pasture, but beside the horses so he-she doesn't get lonly, just remains safe?
I usually do this at first to introduce the horses and after about a month if all is well, introduce them together supervised periods of time.
That will depend entirely on how well they get along when introduced gradually. They could very well bully a mini or the mini could bully them. You could introduce first through a partition, next in hand, and finally loose, but this is always a risk. You have to weigh this for yourself. I had my mini mares in with one large gelding who had a rather aggressive temperament, then another time mini mares in with a large mare. Minis do best with horses of similar sizes. If you do combine sizes, make sure there is plenty of space for the mini to get away if he is chased.
Would a stallion be a problem if I get a young one, with my 3 gueldings (same or different pasture)?
I know it depends on the specific horse, but if some of you want to share some experiences, advice etc it would be great
My short answer, "yes." This again depends on the horses in question and that particular stallion. I have a mini stallion who gets along with other colts/stallions his size, but he bore a grudge against our large Morgan. To the extent that my mini stud pushed through a board to get to the large horse and then chased and bit (attacked) him underneath his belly. Some horses cannot stand a particular horse, similar to human-to-human prejudice.
would a mare be a problem with my 3 boys, the standard horse beeing very stallionish with females... (even as a guelding)?
If he tries to mount other large mares then this would be a big problem. He could hurt a mini mare quite badly, I think.
and finally if the mini could not get along or was not safe with the herd... would he-she be miserable in a pasture besides the others?
I don't believe so. A mini could be quite happy living near large horses!
Sounds like you are well on your way!