Best SA/CA species to pair with sunfish?

I wouldn't put any CA/SA cichlid in a 20 gallon with a sunfish. That tank is too small for all but dwarf cichlids, and the sunfish would probably eat most of those.

Even the sunfish is going to be cramped if it reaches full size. If it's a green sunfish, they have the potential to reach 12", but will probably still end up at 7" or so even if they don't grow to their full potential.

Upgrade your tank first, then think about tankmates. People often come on here and say "I'm upgrading soon" or "I can upgrade later," and then they never do.

I was assuming a 55 can hold 2 maybe even 3 6-7 inch fish?

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Greens don't typically top out at 6-7", and neither do severums. Both can easily reach 10" or better in captivity. Clearly you don't know what you are doing, nor are you interested in taking advice from those who do, so why even ask? Just do whatever floats your boat, but be prepared for the results.

X2, another thing to consider is that natives don't need tropical temperatures, and most cichlids will not do well an unheated tank

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I am not opposed to mixing sunfish and cichlids, and have done so successfully, but I would never try it in less than a 75g, minimum, preferably more. Plus Greens tend to be one of the most aggressive sunfish, so you would need either a very large tank, or just the right fish to pull the balance off. You can definitely have 2 or 3 6-7 inch fish in a 55g - but not if one of them is a green sunfish.

check out some youtube videos of native tanks -you will notice they all share something in common

Once you get a big enough tank ready, I would experiment with a convict. They do the best with panfish.
1 convict not 2. Usually what can't kill the sunfish won't last long though. In a 150 or so with enough hiding spots and caves maybe.

1. Your tank is too small for anything beyond the one sunfish, and even so, he will get cramped soon, by himself.
2. If you only have 2 or 3 fish, then the dominant fish will quickly make short work of the other fish, so you need more fish to spread out the aggression - and your tank, once again, is too small for that.
3. PLEASE don't think of "borrowing from Peter to pay Paul" with your goldfish tank. Maybe in the spring when your goldfish get another home, you can use that tank, but what happens in 6 months when the goldfish need their tank again?

If you don't like the reality, then you should rethink your approach to fishkeeping.

Here's what you need to do: Enjoy your sunfish as-is, by himself - and you won't have to worry about the health of anybody, or maintaining good water quality, you won't have to worry about the goldfish coming back inside, you will have a enjoyable wet pet. It doesn't get better than that. And then, upgrade to a bigger tank when you can, and then start thinking about a couple new fish.
Even in a 55, having only 2 or 3 fish in it, you'll still end up with one.

Greens need a lot of room to establish their territories. I have never had much success keeping them with anything, even in a 55. There was always territorial squables that resulted in serious damage to one fish or the other. Even keeping multiple greens together was a problem. I only keep them in single specimen tanks now, and honestly, don't keep them much anymore because I don't particularly like single specimen tanks. If you do insist on going this route, please prepare for carnage, I've found them to be the second most aggressive fish I've ever kept. The first was tetraodon lineatus. Killed everything.
If you want community tanks, may I suggest going with a different centrarchid? Dwarf sunfishes perhaps? Fliers, Blue spotted, pygmy, all do well in groups, and stay small enough to live in a 20 gallon for quite some time.