I need some build ideas

Having flown it a couple of times I really hate the GR-75, as the whole "not having guns" thing is a real turn off (especially when empire and now scum have such awesome small epic ships that do have a hardpoint). That said, the best way I've seen it used capitalises on its jam action, so something like this comes to mind as a way to make it (sort of) useful:

GR-75 Medium Transport (53) - GR-75 Medium Transport
Raymus Antilles (6), Intelligence Agent (1), Frequency Jammer (4), Dutyfree (2), Combat Retrofit (10)

Ahsoka Tano (21) - TIE Fighter
Veteran Instincts (1), Sabine's Masterpiece (1), Captured TIE (1), Black Market Slicer Tools (1)

Combat retrofit is optional, it just keeps it alive and stressing longer. Everything else it meant to improve its ability to cause stress, while Ahsoka, and perhaps a YT with snuggling compartment and BMST try and deal damage.

I have seen it used as a battering ram to decent effect.

GR-75 Medium Transport: GR-75 Medium Transport (30)
Construction Droid (3)
· Quantum Storm (+1 Energy) (4)
Engine Booster (3)
Backup Shield Generator (3)
-- TOTAL ------- 43p. --

Backup Shield Generator is Optional, as it can go with Quantum Storm decently well. Although if you're running out of energy a lot, then you might just want to drop it and keep that energy for Engine Booster. A Shield Technician may be helpful, but that's up to you.

Another option is a massive support boat. comms boosters, targeting coordinator, automated protocols, and em emitter comes to mind.

Are there any guides around for fielding rebel epic fleets?

I love the GR75. A friend even goes so far as saying if he could take it in a standard list, he would. The key is to build it light and focused for a specific support task. You build it point heavy, it becomes a real blow to your forces when it dies. This is a dedicated support ship. Style it to support your particular fleet.

Some build ideas:

• Bright Hope, Automated Protocols, Comms Booster, Targeting Coordinator (or any combo of two of Comms Booster/Targetting Coordinator - The goal here is to give actions to the rest of your force. First action can be to coordinate or jam, but always reinforce the shield facing the enemy.
• Duty Free, Frequency Jammer, Automated Protocols - Stressbot on steroids! 41 points - jam an enemy ship out to range 3, potentially adding one stress to another ship at range 1 of that one, and then reinforce your shields!
• Slicer Tools, Automated Protocols - 42 points - wingmate for Duty Free if you have two. Deal damage to ships with stress, then reinforce those shields.
• Engine Booster - "Hi. I'm a train. You're a bug on the track. You're going to have a bad day."
• Throw Toryn Farr and Automated Protocols on one and watch all those aces who depend on tokens stay as far away from your transport as possible!

Stay away from combat retrofit... it's a lot of points for very little gain. It's like tossing shield upgrade and R2D2 on Biggs, really.

Edit: If you've noticed a trend - automated protocols is a HUGE helper for this particular ship.