Horses placed together will always have a hierarchy which they will have to work out. A mare will always be domant over a gelding. That is the way they are in the wild, and it is their instinct. Some horses have likes and dislikes, there really is no way to know until you introduce them.
A horse that does not have a companion is lonely, but that doesn't mean he/she can't deal with it. My mare is alone, she has me, and there are three others she can see, but not easily. They are in a neighbor's yard. Christy does quite well.
As long as your horse will lay down, stretch out and sleep, he is ok. If he doesn't do this, then you need to get a companion or he will have sleep deprivation.
There is a herd instinct which all horses have. The mare is the boss, even over the stallion. Then, among mares there is an order that has to be established. One will be the boss, the others will follow, but again, in a hierarchy order. They work it out by biting, kicking, and aggression. Once the order is worked out, they usually do fine. With an occasional minor problem.
If you get another horse, don't put it immediately in the same pen, keep it close, but not touching. This way they can become acquainted before being put together. It is actually best if they can be kept seperate, close, but seperate. That way you can avoid the dominance issue and at the same time they will not be alone. If you do put them together make sure you have two seperate feeding places and put feed at both places. That way when the dominant horse runs the other away, the other can still eat. :)
As for gelding vs. mare... It depends on you and which you would prefer. Most geldings get along well, they will work out who is boss, and then settle down (well, mostly
.) The mare will be the boss, so there might be a bit of fighting at the beginning, however most geldings will accept the mare as boss without a problem. So, it really depends on what you want.
Hope this helps.