Yeah, I've only found small chunks of tin, iron and copper on those sort of mountain quarry type areas, the ones where the surface material is just stone and ore. After a number of hours I only just scraped together my first iron pick. I found a mineshaft, two ravines and a lava lake, but I only ever came across large amounts of the exotic ores like magnetite, pitchblende and the orange cerium ore which I forgot the name of. No trace of any massive veins of copper, tin or iron for me =n= Then again I haven't been able to do much digging (but it's sooooo boriiing) Maybe I should start over on a fresh seed if anyone has one where a vein is guaranteed to spawn. That's the one thing I hate about super tough mods. It's not that some of the cool machines are disabled or the crafting recipies are different, it's the fact that it puts you in the position where you just have to grind and grind just to reach the bottom level of stuff.
Yeah, Gregtech oregen is something you either love it, or you hate it. For me, I love it so much that I can't play any other modpacks which have what I call "vanilla oregen", where you can literally find every ore in every chunk. That's not at all realistic. But GT oregen is very realistic. Yes, ore veins can be difficult to find, but for me that's a good thing, not a bad thing... because I've always felt the game needed more reasons to make the player get out and explore the world. But there's a reward for it being more difficult to locate ores - the benefit is that when you DO find a vein, you're all set for a long time and won't have to find another vein of that ore for a while.
For me, I dig tunnels under my base at exactly 3 chunks apart horizontally, and I make a criss-cross grid of such tunnels. I make a grid at elevation 11, elevation 25, and elevation 45, which hits the middle range of most GT ores. It would be very unlikely to miss a GT vein using this mining technique (and eventually you can make the IC2 OV Scanner item to check and see if one did get missed once you reach HV era). Yes its a lot of digging, but its well worth it to scout out the veins. I don't mine every vein I find right away, most of them I simply make a waypoint and come back when I can run the GT Advanced Miner II on the ores, for additional bonus quantity of raw materials.
To get started, walk the surface until you find a tin vein, and yes it might take you a while. But collect the small ores as you go, for early game iron (just like you said you are doing), and only settle when you find a tin vein. Then, make your base and dig down to elevation 25. That's the middle elevation for iron ore (which will actually be the Brown and Yellow Limonite ore vein, those smelt to iron in a vanilla furnace), and for copper (which will be the Chalcopyrite vein, which smelts to copper in a vanilla furnace). Make a Quartz Grindstone with the Wooden Crank to double your early game ores until you can achieve the bronze age Macerator. Hope that helps you a bit, once you get going in the pack it does get better.