Rebels are actually the strongest faction currently, in my opinion, being the only ones i can beat Expert difficulty with, and relatively easy.

Always remember, small ships are king. Rebels have the best small ships.
Rebels start with some of their most cost effective units at tech 0 and the other ones are not far up either.

Build Z-95 and X-wings. They easily allow you to get fighter superiority.

Build all the Cr90 you can. They are THE best corvette in the game, allowing you to easily beat all other corvettes and frigates like the Carrack, which is unable to hit corvettes. Cr90 is also very powerful against fighters and bombers. It also hard counters things like the Imperial Stard Destroyer since it lacks anti-corvette weapons. It just takes a while to chew through the health.
Focus enemy ships with Assault Missiles and Heavy Lasercannons first to allow you corvettes free reign over the battlefield. No other weapon can really hurt it.

Other ships of note are:
The Marauder Corvette, the best artillery ship in the game. Gives the Rebels access to Ion Cannons which their fleets otherwise lack. Overall gives a pretty good punch for its cost. And because it is a corvette, it is essentially immune to Turbolasers, like all corvettes. Keep it safe from strike craft, Assault Missiles and Lasercannons and it will just sit there, dish out lots of damage while taking none in return due to its tiny size class.

MC40 Frigate. small and powerful. A pile of medium turbolasers and plasma torpedos on a small, cheap ship that just doesn't die. This ship is incredibly tough and dishes out a ton of damage against other ships. And super cheap to boot. Only downside is that it has essentially no anti-fighter weapons. But you are the rebels. If you lose the fighter-war, something has gone terribly wrong.

MC80 Justice. Versatile, flexible, cheap. While they can't stand up to a star destroyer in a straight fight, they are much more cost effective. They can very well beat up anything smaller, removing the support that an ISD needs to survive. It also brings 9 whole squadrons into the fight which is just insane. They are also cheap enough to mass produce towards the end of early game.

For ground
I mostly just spam cheap infantry. Some heavy tanks to support and focus down enemy vehicles. The transport vehicle with the ion cannon is also extremely helpful for disabling and stunlocking stronger enemy vehicles, or even turrets.

Galactic Map.
First thing, sell all the ioncannons, shield generators and turbolasers you can find on the planets. They are pointless and only cost money. Use the money to upgrade your offensive forces and economy instead.
In the southwest around hoth, just grab Bespin and build up some light to medium defences in the area. some infantry on the ground, a bunch of corvettes and max level stations in space. And then just ignore this area.
South east around Kamino, grab your forces and send/retreat them towards the Kashyyk area. Collect all the forces there and push through Kessel to the north east.

North East with Mon Calamari as your capital will be your main area of action. Team everything up into a doomstack and wipe out the black sun. they have not enough forces to stand up to you. This gives you extra breathing space, planets to build Manufactures on for that sweet moneys, also build a pile of Gr75 transports for even more moneys.
It is very easy to make a silly amount of money as the rebels. Somehow they make more money than the empire.
This should also had you conquer the planets needed to obtain Ackbar.
Continue to roflstomp around the map as nothing can defeat your fleet now, at least this early in the game.
Keep attacking. Don't stop. Always attacking. Build more Cr90s, X-wings, Mc40s and infantry etc. Keep up the pressure.

Thx you very much. I'll try that.

I agree all he said especially the part about not having the economy to do anything. You are not strong enough to take a single planet thru anything other then the revolt system. Focusing on a fleet to conquer something means no economic growth or money to complete build mission. So you have to choose do i complete build missions and let pirate rescue and planet capture missions go or do i build a fleet and say screw build missions. Either way I am taking a hit to my Rebel support. Hard is fun but this is beyond hard.
Uping the amount of money from smugglers would be a good start.

I can fix this really quickly for you.

The Imps have nothing that can live with the X-Wing and they're a better strike fighter in this mod than previous. Build and micromanage X-wings. 24 of them win. 40 of them makes sure you win.

Charge the battlefield with X-Wings and don't build anything else.

I have done a bit of recording and this is how i end up rather quickly in a state where i have essentially won the game as Rebels. At the end of the second video, you just build up the conquered worlds a bit more and reach about 13k income. Then you just build ships and ground units while turning the entire map red.
No techs needed, really.

Might not work once the next update hits.

https://youtu.be/1PxhFfN6QUw

https://youtu.be/pi-QalhNVBY