@MigukNamja and
@Tyriael_Soban covered pretty much everything already, but I figured I'd chime in too.
Steam These produce more than enough steam, but I am extremely adverse to setting up charcoal farms for power, and I can't think of any other way of running the boiler without spending all my time mining coal. Also, If I was going to do this I'm not what would be best to use to automate the tree farming. EnderIO Farms are a bit pricy powerwise, and because of reasons I am forbidding myself from using thaumcraft for the moment. Are there other options in this pack?
Sadly I have a hard time picturing myself getting to any point in this pack without steam. There are several mods providing tree-cutting toys, and with with high-efficiency railcraft boilers and high-output GT boilers, its just too powerful to ignore. If you're going to go sans-steam, its going to be an interesting challenge for you.
Diesel generators - I have no oil anywhere even remotely near my base, and without some form of tesseracts (which seem to have been removed from EnderIO in this pack) I do not see a way to get it to my base easily.
Your best bet would be Drums, but these aren't cheap in steel. A good oil well can easily fill 30-40 drums iirc, so be prepared to make multiple trips and set up a chunkloader.
That oil well will produce enough power to last you a very, very long time.
Big Reactors - As I understand it, these produce RF only which cannot be transformed to GT EU or IC2 EU?
Miguk touched on this, but yeah, you have options. The Energy Storage Module (or Cluster) from GC gives the best ratio.
IC2 Nuclear Reactor - This functions in this pack, right? And can be converted to GT EU with a transformer? This is the most appealing option I have found so far, but I was really hoping to stick purely to GT as much as possible until I reach endgame. Is there a GT nuclear system? I cannot seem to find it in NEI but I know have seen it mentioned before somewhere.
Gregtech does not have a nuclear fission system, and the nuclear fusion system has not yet been re-implemented in GT5. (fingers crossed though, there's rumours it has his attention these days)
Solar Panels (GT, not the IC2 one) - I'm confused by this one. I can only find a recipe for the simplest one in NEI. I assume then that all the bigger ones are disabled in this pack? Is that correct? And the simplest one isn't really going to be able to do much is it? (I assume you'd have to build a zillion of them and it would risk seriously lagging the server)
You're right, the "buff" ones are disabled. I sometimes use the small ones for outlying farms and such, but otherwise they're completely avoidable.
- Can I power my Genedustry machines from RF?
Last I checked you could not. Interestingly this is the only mod that I've ever seen take GT power directly. I have no idea if the RF thing is intentional.
SUGGESTION
One thing totally missed above was IC2's interesting biochaff power system. If you want to get into any kind of farming, you can produce biochaff, convert it to biofuel, and get power from that. Of course, that's not a GT solution, but the modpack provides plenty of options for a reason
About Big Reactors, I have recently made a pretty good setup. 13x13x4 in size, producing steam (you need a lot of water, I have used 23 Ender IO reservoirs), feeding into 3 RC turbines. From there you can use lossles IC2 HV cable to wherever you want, then MFSU > GT transformer, and you have HV power.
Note that you need quite a bit of resources for this, including being able to make tungstensteel for the reactor.
Sadly you can't make a closed water loop if you're using RC turbines.
My 11x11x2 (interior) reactor is only using 4 reservoirs, and I doubt I need all of them.
Since I only have one turbine at the moment, the reactor is operating with rods mostly throttled (90% closed). Even so its producing 7000 RF/T with substandard electric coils. Once its upgraded to Fluxed Electrum and/or Ludicrite, it will be producing at least twice that. And since this reactor can feed 5 more turbines on top of that, we can confidently say 14000RF/t * 6 = 84000 RF/T.
Convert that to EU via Galacticraft and you have 21000 eu/t, and no rotor replacing
On the downside (?), these turbines are absolutely massive...