I have 5 boys, 3 together and two together who live next door.
We started with two, Spike and Gizmo then adopted Stripes but Stripes and Gizmo made it very clear there was no way they were going to share Spike so Stripes became the next door neighbour.
When we lost Gizmo, Spike and Stripes moved in together, we sadly lost Stripes this summer so ended up in the same position as you with just Spike (a 2 year old, un neutered)
There was no way I was going to risk having him neutered so we took him to Wheek and Squeak rescue in Southampton for some boar dating, originally just for him to pick a friend. (we ended up bringing another 4 home, but that's another story!)
We tried Spike with Hector (21 months) and they got on great, I'd still been thinking about a pig called Bob (19 months) so we added him to the group. It only worked because Spike and Bob are so laid back and Hector is the bouncy one! They worked out who was boss straight away so it meant they were relaxed with each other.
I think the problems come when a young pig goes through the teenage stage and tries for top pig. As they were all a bit older it was much smoother. They stayed at the rescue for a week so Anna could observe them and make sure they had bonded. When they came home they did have a few moments of Hector getting a little carried away and trying to be boss but Bob put him in his place and after a week they settled down really well.
So I've experienced both sides really. It is all down to personality, Gizmo was very lively, Spike very laid back and Stripes very nervous so Gizmo was just too much for him.
Bob and Spike and the most chilled out pigs ever and clearly love each other, within a minute of putting them together they were washing each others faces. Hector is like the naughty little brother!
I really hope you can find a combination that work. I think adding an existing pair might be harder as they will already have a strong bond. Maybe three individuals. I think there's definately only room for one fiesty one in a trio!
Good luck!