There is a Default Config Pack on the GT6 Download Page, you should take a look at it.
I so hate how Quote splitting doesnt work anymore on the Forums...
Underground Biomes Constructs - Aesthetics and breaking the monotony of mining stone.
Underground Biomes is irrelevant by now, GT6 has its own better Underground Biomes now. You can still install Underground Biomes, but I would turn UBs Worldgen off, because GT6 will generate the UB Rocks with its own Generator when installed.
Better Records
This one sounds like something I should add GT6 Shelf compatibility for. Could you link it to me?
ATG (Alternate Terrain Generation)
I didn't test GT6 with that one in a while, lets hope it still works. I know I fixed some RTG Compat a few months ago, so maybe that fixed anything in similar Mods.
ExtraBiomes XL - If better alternatives exist, will be replaced.
Dunno if better alternatives exist, maybe BiomesO'Plenty, but you can have both of them at once. It's all about ATG Compatibility I guess.
ZTones - Nice assortment of blocks.
Another one I would like to take a look at. Link pls.
JABBA - Possibly unbalanced?
Not needed at all anymore, GT6 has its own Barrels now, including a Block that lets you automatically fill all Barrels in a Wall when you rightclick it, while not allowing Pipes to connect with that "Controller", meaning you do have to pipe directly into all Barrels instead of one central Controller. Also GT6 Barrels automatically convert tiny and small Piles of Dust to regular Dust and Dust Blocks, and vice versa.
Iron Chests
Yep GT6 has its own Chests since ages.