ROMs Contimplating a Samsung Transform & a custom rom

[searcherrr]

I'm a developer of web applications, but I've never delved into the mobile apps stuff till now.

I realize a new ROM will basically rewrite/reflash the phone's OS, but what else does that include?

Does the ROM include the functionality of the PHONE itself? IE: Calling, text messaging, picture mail etc... or is that separate from the ROM and hard coded for each specific phone?

I saw someone on YouTube do a custom ROM for a Samsung Transform, but I have not seen it anywhere else. Anyone know?

Is there any reliable Custom ROM "depot" online or is it just luck of the draw wherever you find it type-of-thing?

What would it take to build my own custom ROM? Would I have to have the stock ROM and then backwards engineer it? If so, what SDK would I use to do something like that? What toolset?

If this phone doesn't have any custom ROMs available, what about rooting it? Anyone rooted a Samsung Transform?

Can you put ANY version of Android on ANY phone that has had Android installed on it, or will this depend on the hardware of the phone as to which version can be loaded?