So far my team is Do you have any advice on a 4th move for Slaking and my third pokemon?RNG manip is, in my opinion, totally the way to go. Yes it takes more time initially, but once you get the hang of it, it doesn't end up taking very long and you get stronger mons, which sometimes makes a big difference. Plus it's kinda fun. For Latios, I would compare your Latios' stats against what a Starmie would be. Due to a glitch, Pokemon caught while roaming in this generation have IVs of zero in everything except attack, which is 5 or less, and HP which is the normal 0-31. Mild is not a bad nature when considering you could've ended up with something way worse. Plug in some numbers here against opponents you might face and see the difference yourself: https://turskain.github.io/ (switch it to ADV). Also, consider the synergy. Slaking is weak to fighting, which both of the psychics resist, and metagross is weak to fire and ground, which both resist fire, but Latios is immune to ground while Starmie is neutral. That being said, if the attacker is ground type, then Starmie's stronger surf might come in handy. There's always tradeoffs.
For breeding/RNG abuse, I have a video here that goes over this whole process of start to finish of how I get a mon I want:it's very long, so feel free to skim or not watch, but this is like an info dump of like 80% of what I know about breeding/EV training/RNG manip optimizations specifically in Emerald.
I think your team will be plenty strong and I think it's worth obtaining the team and testing at this point. There's only so much you can do with theory alone- go play with it, pay attention to your losses (or maybe you only win and get all Golds! yay!) and come back and people can help you workshop it more. Also, even if you have a great team, if you don't know why it's good and can't wield it effectively, you probably won't make it to Gold, so consider after you train your Pokemon, train yourself a bit too by playing some runs and getting used to the Frontier. Good luck!
SLAKOTH Adamant IVs 31/31/31/28/31/31 - Choice Band - EVs 4/252/0/0/0/0/252
Return
Earthquake
Shadow Ball
?? (What should I put here? Rock Slide? Brick break?)
METAGROSS Adamant IVs 27/25/29/11/28/31 - Choice Band - EVs 12/252/0/0/0/244
Meteor Mash (the accuracy on this move really pisses me off)
Earthquake
Shadow Ball
Explosion
Latios Mild IVs 18/22/8/26/19/24 ??? (Will take suggestions on a different 3rd pokemon here if anyone has please).