Hello!
Getting your pigs neutered will not change any territorial issues. And you're absolutely fine with pairing him with a baby, it may even make the initial introduction go smoother. Be prepared that there may be a few squabbles once the baby enters puberty though!
The Introduction Process:
1) Allow Felix to help choose his new friend. Many rescues will allow "speed dating" so you can see how well he gets on with another. Things will be a bit different once the newcomer is in "his" cage, but initial reactions are good to base your decision off of!
2) Allow them to meet on neutral territory. For example, a playpen or the bathtub. Keep them in seeing and hearing distance while doing this, with a towel and some heavy duty gloves in hands (just in case a fight breaks out, you want to be prepared!). If they're getting on fine, start cleaning out the main cage.
3) Clean out the main cage! You want it completely scrubbed down, you want the scent to be as neutral as physically possible.
-If you use disposable bedding, change it out completely
-If you use fleece, throw it in the washer and dryer. Along with it, throw in anything else that can go in the washer (snuggle sacs, cuddle cups, other beds/stuffed toys, etc)
-Wipe down EVERYTHING plastic (a lot of people recommend with a vinegar solution). The plastic of the cage bottom, any hideaways, any ball toys, etc.
-Because wooden things are harder to clean, simple remove them from the cage. Introduce entirely fresh chew toys. If you have wooden houses, just leave them out until the boys are both settled into the new cages (I'd give it at least a week to be totally safe)
-Put water bottles, food bowls, hay racks, etc... either through the dish washer, or hand clean them.
-Put fresh water, food and hay into the cage.
Once the cage is done, start up a bath for your pigs. Wash them together. This is called a "buddy bath". Not only does it neutralize their scent, but a lot of pigs find baths a wee bit scary, so it's not uncommon for the two to go to one another for comfort.
Once the cage and pigs are neutralized, put them into the shared cage together. Do not separate again after this. I can't stress this enough - Do not separate them again unless actual violence breaks out (ie: blood shed). Humping, nipping, chasing, etc, are all completely normal behaviors. You will most likely see some of this things happen because it's how guinea pigs settle dominance. It's a bit scary to watch, but it's normal, and you have to just let it run its course.
Good luck!
