Young Link really isn't very good in most relevant matchups. If you want to win against most of the characters in the S and A tiers, you really have to outplay and outsmart your opponent bcause Young Link isn't a very good character.
I'm not sure how Young Link would be as a secondary to a Falco main because Falco can manage all of the stuff that YL can and do it better. The only common matchups in which you might want to consider YL over Falco are versus Jigglypuff, Peach, maybe Ice Climbers, maybe Samus, and maybe Luigi. Even then, Young Lnk struggles in these matchups. For example, I would personally pick Falco over YL against Jigglypuff and Samus because they are both very good at threatening your space. You and Samus both excel in the mid range, but Samus has better tools in the mid range than you do, so playing against her can be troublesome. Jigglypuff is very good at threatening your space so that you can be punished for throwing projectiles.
Young Link is horrible at killing, so he's really only good against light or floaty characters. Against characters like Fox and Falco, you have to rely on gimping (which Young Link is pretty good at) because Fox and Falco outclass YL so hard that there's not really that much else you can do. Just use Falco for those matchups. Since you main Falco, there's no reason to go YL against non-floaties.
Young Link has a few killing options against floaties, the most popular of which is bomb --> dair. Practice that. Learn to time the dair so that it hits right after the bomb does so they can't SDI the explosion and escape but early enough so that the dair doesn't get negated by one of the bomb explosion's hitboxes. Usually the timing on this is very lenient. All you really need to do for it to be useful is to learn to time it so they aren't able to escape the setup.
Young Link sucks, so you need to learn to play tricky. Practice lots of tech skill, but only practice useful tech skill. Don't learn crazy, situational tech like ledge-cancelled z-air (which Axe uses a lot) until your movement is so good that you never mess it up. You don't have much room for error with Young Link since he gets punished very hard. When you have good movement, learn to move around in tricky ways. You don't want to spam tech skill; you want to make sure the opponent has trouble tracking you and knowing where you'll be. Young Link is fast, so this works very well. Just make sure you don't techskill yourself into a bad position.
Learn tricky projectile stuff. I love to throw bombs straight up because it threatens some space. I love to drop bombs on platforms so I can quickly grab them and use them. I love to ricochet projectiles off of the ground because the angle at which the boomerang bounces up covers a ton of space and often tricks the opponent. Make sure to mix things up and condition your opponent so they keep falling into your tricks.
Learn recovery mixups. YL is easy to edgeguard, so you really have to outsmart your opponent to get back onto stage. Aside from an okay up B, you also have a decent hookshot and a walljump. A favorite of mine is to hookshot the stage without using my double jump, wall jump off the stage, and double jump back on while protecting myself with b-air.
Watch other YL players. Axe is very good and pretty popular, but he doesn't really camp like other YL players do. Watch the likes of Armada, Laijin, and Rhan.
I feel like I forgot some stuff, but here's what I came up with off the top of my head.