[CFG/Modeling] Big parts development advice needed

[Rev]

First of all, hello to everybody as this is my first post here.

With the introduction of new celestial bodies, it became obvious that I have to build bigger and more powerful rockets to get anywhere, so I decided to make required parts by myself.

More screenshots

Parts

So far I have made a 8m high fuel tank, 2m adapter (2.5m to 5m) with built-in fuel tank and quad coupler (5m to 4 x 2.5m). They are not 100% ready to be released, because I have to face a few problems first. My experiance with 3d modelling and texturing is extremly limited so my parts are far from perfect.

1. Any rocket with 5x8 fuel tank has a tendency to spin out of control which caused deaths of many innocent Kerbals. Considering that the 5x8 fuel tank weights 144 it equals 8 stock 2.5m long tanks, I've build a rocket using quadcoupler with 4*4 stock tanks. 4*4 setup works like a dream, it's perfectly stable. What can make 5x8 fuel tank spin like mad?

2. All parts are acting quite wierd midflight. They're sliding on the surface of parts attached below. It's not gamebreaking, but looks awful. Is it possible to do something about that?

3. Last but not least, question about collider-mesh size. Parts with collider-meshes as big as render-meshes cannot be connected with anything else unless allowCollision parameter is set to 1. I believe that doing that is only a mere workaround, not a proper way to do it. Is making collider-meshes a little (0.01 for example) smaller is a way to go?

If you have got any other helpful advice, or comments about the whole 5-m parts idea, I'll gladly learn your view.

[SpannerMonkey(smce)]

Excellent first parts sort of puts my first part where it belongs on the scrapheap, good luck with the adventure

[mivanit]

making collision meshes slightly smaller worked for me, but then again i have very little experience

looks awesome for now, if a bit un-creative (NOT trying to be depressing-keep up the good work!)

[Chromium]

As I've been creating my own parts, I've ended up going the "scale collision meshes to 99%" route, which I prefer to allowCollision 0 to avoid having rockets with (intentionally or accidentally) intersecting parts. Looking forward to seeing someone more experienced answer the other questions, though.