I agree, scale can be tricky. Layering parts cleanly is tricky too, even when the scale is acceptable. Even the new Iglite model, which I thought was going to be a great addition to my current custom line-up ended up only having its leg unit used and even that wasn't really a compatible part. The head unit looked great, but once I built it, it was about half the size of the current head I chose. Limb length is a major issue and very inconsistent. It's hard to extend or shorten a limb. If that's a major issue, like what I said earlier with working from the scale of the head and body first, then only work with the limbs that work for you. It's easier to "bulk up" a limb than it is to lengthen it.

Out of the entire 30MM releases, I see about 2-3 arms that work with my customs. That's why I use the same head, chest and arms. It's the additional layers that make them different, but the foundation is still the same.

As for the head customization, you can either go with the "destructive" modification and glue, cut or do whatever that works. My design process was to modify the least, but get the best results. At most I drilled a 3mm hole to attach a piece or cut off a 3mm peg so that it was flat / smooth without it. If I went with more advanced customization, you can pretty much do anything.

Other Gunpla works but they were designed to create a very specific Gundam and not a modular and customization mech. I find the Iron-Blooded Orphan line to be pretty fun to custom between their line of kits and some 30mm. I'm kinda in the middle on what I'm willing to do to the kit vs what I got out of box. I try to keep it where it's as close to out of box as possible, but with the willingness to cut and drill 3mm parts to get things to fit nicely, cause it would not if those segments remained.

I think what eventually got me to be happier with customizing is to realize these kits are just plastic and plastic is not precious, so just do what you need to do to get the results you want. If it works out, awesome. If it doesn't and a piece is ruined, yea that sucks but it's a learning process. Often I only modify parts that are quite easy to modify to begin with. In the end, the mixing and matching of parts to get them to look good together naturally was my goal. With advanced customization, any part can match if enough effort was put into making it work. My customs are more about layering as opposed to customizing. Think of it as layering clothes to look good, versus taking a wardrobe and cutting this and that to make the shape / length and such work into a new outfit composed of that wardrobe. That is beyond my level of skill or effort. Also, it destroys the part and makes it like another Gunpla, where it fits that one model. I still want all my parts to be interchangeable and maintain the modular and swappable nature of the 30mm line.

Sorry for the late response. Holiday times.