Good for you for wanting to rescue another baby, and I think it's a great idea to give Momo a companion. You know he has been around other cats, that is a good thing. Sometimes it doesn't always work out that they hit it off and become buddies right away, so I wouldn't be toodisappointed if it takes a little time. I do think a younger cat is a good idea, I think kittens are usually better accepted by another cat rather than an adult.
In my case, it worked out great. I had Taffy, who was 5 at the time, and we rescued a little stray from the woods, a little black kitty who desperately needed saving, and he was estimated to be about 5 months old....not a tiny kitten, but definitely still a kitten. He was already 8 pounds. His name is Stephano, and he is a hoot.
The first thing you should do if you get another kitty, DO NOT just bring the new cat home and let him have the run of the house, that is very traumatic for the new cat, and the old cat, and that is a mistake many people make. You should put new kitty in his "safe" room, a bedroom preferably. Let them start checking each other out under the door, and believe me, they will. Start feeding them near the door, together, and maybe give some treats. After they've had at least several days, if not a week, of checking each other out under the door, crack the door so they can see each other, but don't let them out together yet. Expect some hissing and growling from either of them, and that is OK and to be expected. ACT NEUTRAL. Don't punish anyone for the hissing growling, you just act like you are indifferent to the attitudes. I can't stress that enough, your calmness is very important. If that all goes well with the door cracked, and you feel like you are ready to do a face to face, you just have to see how it goes. Even my cats, who hit it off within a day of being out together, there was some hissing and growling on Taffy's part, every time he tried to approach Stephano, Taffy growled and poor Stephano just looked crushed. But within a day, Stephano tried to play with Taffy, who would back off, and finally Stephano just (playfully) jumped at him and literally rolled him, lol, it was hysterical. And Taffy was NOT amused, but he then realized that this clown was not going to hurt him, and they became very close within a day. LIke I said, this does not always happen this easily, more times than not, it involves a lot more hissing, growling, even some smacking...that could happen too. But even if it does, do not react. If it goes badly, just put new kitty back in his safe room, or if Momo is annoyed but not going after him and just wanders over to sit in a chair and pout, you can let them stay out (supervised) and see how it goes. I'd put the new kitty up in his own room at night, for a while, until you know that they are getting along, that is really important. For one thing, kittens get into all sorts of trouble at night, can chew things, as well as they could fight in the middle of the night, so putting new kitty up for a while at night is a good idea. Even though Taffy and Stephnao hit it off right away, Stephano was a young, rambunctious little guy who got on Taffy's nerves after a while, and I would give them breaks from each other, put Stephano up in his room and leave Taffy out to sleep in his chair in peace without being pounced on to play with. Also, Stephano was hassling Taffy, just trying to play with him when he would go to the litter box, and Taffy ended up having some accidents, which we did remedy by adding more litter boxes around the house. You need at least two, if not three. I know that sounds like a lot, but trust me, you want to have designated places for them to go so that they always have a place to go nearby and they don't resort to using your couch or some other spot. That is really important, and kind of pay attention to what goes on there, and see if one is bothering the other while they are trying to do their business.
My sister and my husband's aunt have another way of doing it, they just let them out together after the time spent in the safe room, and ignore any attitudes. I don't really like that way, but I know sometimes it does work well to just not make a big deal out of any attitudes they are giving each other and act like it's just an ordinary day, nothing new taking place here. I believe your attitude has a lot to do with how well this process goes.
Just don't expect too much too fast. You know that Momo was OK in the shelter being around other cats, that is a good sign. You never know until you try. I think a kitty under a year old is a great idea, still young enough to be funny and playful, and not old and set in their ways yet. If it does work out that they become buddies, that is awesome. Or if they just learn to tolerate each other and are not playful together, that's OK too. I have added another stray to the mix, last February, a little tabby named Pippa, and she does NOT want to play with Stephano or Taffy, but she likes being around them. They will lay next to each other on the bed or the cat tree, but they don't really snuggle. And even though Pippa does not want to play with Stephano, he still chases her up and down the hall sometimes and gets her playing anyway, and they have actually started being quite playful with each other. Somtimes he's a butt and smacks her, but she stands up for herself even though she is literally half his size.
Good for you for wanting to add another kitty! You will be saving another life, and hopefully giving Momo a companion so he won't be alone while you are gone. Just make sure you give Momo lots and lots of extra love and attention after new kitty comes along. He still needs to feel like he is the king of the castle and that you still adore him. Also, get a toy like "da bird", if you don't have one of those, most cats go insane over that one, and play with them together. We have at least two play sessions a day to keep Stephano from going to stir crazy and then bothering Pip. When he starts to focus on her, and starts bugging her, we get out da bird and they boy go crazy and play for a while, and then they are worn out and he is no longer focused on bugging her.
Can't wait to hear how it goes