Any tips, tricks, or hints for a guy that usually gets his butt kicked in original Sins?
Well, going from the original Sins to Rebellion means you also not only get the new stuff from Rebellion, but the earlier Entrenchment and Diplomacy expansions as well.
One of the big problems with the original Sins was that defenses quickly became inadequate in the late game, where large fleets could easily take out a planet even with maxed tactical slots. Entrenchment largely fixes this by adding a new defense tree, with upgrade for existing defenses and a powerful new unit type called a Starbase. These are customizeable space stations that when fully upgraded can be a major pain for even late game fleets to take out, especially when supported by other defenses like repair platforms. Basically, you can now fortify a line against one enemy and be reasonable confident it will hold while your main fleet goes on the offensive in the other direction.
Diplomacy as you might guess adds more diplomatic options. It also adds a new diplomatic related research tree, as well as a new means of building up relations with an ally, the envoy cruiser. The envoy cruiser will slowly increase relations with the player who owns whatever planet it is orbiting, and you can get useful abilities for it to help improve your ally's planets. With higher relationship scores you can use a new feature called "pacts", which lets too players work together on technology to give powerful benefits to both players. There was also a fair amount of miscellaneous improvements in this patch, like a few new artifacts, off the top of my head.
But Rebellion is by far the largest of the three expansions. Major features from their include.
- The three races have been split up into a pair of subfactions, each with its own unique specialized play style, new techs and...
- Their own unique titan, an extremely powerful new unit type which serves as a one of a kind flagship for your fleet.
- A new capitalship for each of the three races.
- A new smaller unit type, the Corvette, which is fast, powerful against long range frigates, siege frigates and titans, and can passively debuff certain attributes of enemy ships it attacks.
- Toggleable victory modes!
- New maps!
I believe the GoG version also comes with the two smaller DLC for Rebellion, which adds.
- A bunch of new planet types and bonuses (Forbidden Worlds).
- Planet Specialization into social or industrial worlds (Forbidden Worlds).
- New Stars and other uncolonizeable planet types (Stellar Phenomena)
- Random Events (Stellar Phenomena)
I know that's more about what is new for you versus tips, but knowing it exists is always the first step. 
