I really love this game (Rebellion) but it seems to be abandoned by the developers and the community?
The community has sadly moved to the steam forums and moddb for mods.
I found a few old and inaccurate guides that are just not really useful since they are outdated.
Again, its all on steam. Though feel free to ask specific questions here.
How is the current (latest patch) balance in the Rebellion?
Are any units overpowered or some units too weak/useless?
What are the overall strengths and weaknesses of various races/factions (loyalists/rebels)?
On a faction basis its actually one of the best balanced version ever. In single player it really doesn't matter at all. TEC Loyalists and Advent Loyalists might be slightly on the weaker side in multiplayer, but I've seen people win with anyone. There's certainly some ships that are way better than others, but overall their spread out among the different factions fairly evenly. Vasari and Advent might still have slightly more but they're also harder to learn to play.
Will there be any more DLCs or expansions coming for Sins of a Solar Empire?
I have the Rebellion which includes everything ever made for SoaSE, but i was hoping for something new and fresh to come out soon?
Probably not, Stardock says there's not enough room to add any more DLC without hitting the 32bit 2GB RAM limit. Many mods already require using something called Large Address Aware to increase the RAM available to the game.
Are there any noteworthy addons or mods for the Rebellion?
Perhaps a custom AI that makes the computer fight/build better and be a better enemy (or ally)?
Uh, tons. Honestly the mod community for this game will be the longest living thing about it, there's no doubt about that. I suspect it will still be kicking even after Sins 2 is released. Whenever that is, if it happens. We know nothing.
Sadly the AI is the one thing that isn't that moddable. However, there are mods for almost every Sci-fi thing you can imagine, plus plenty that change up the gameplay a bit. A few do make the AI quite hard, but its really not that different than playing on harder difficulty levels. Its still AI cheats, just different types.