Addon file system structuring

I'm sure this has probably come up at some point but I couldn't find anything on it in my searches...so sorry if this is oft-repeated.

Hypothetical scenario: I've got a clock addon. Due to my anal-retentiveness, it's separated into the following files:

clock.lua --All the frames & appearance-related stuff defined here.

thingus.lua --The actual timing function.

clock.toc --Self-explanatory.


1. Is there any reason not to do this?

2. When loading multiple Lua files in a single addon, are they simply appended to the preceeding file in-order as defined in the ToC? Or are they handled as "separate addons"? Example with two files:

local thingus = somevalue

function dostuff(when)
    print(thingus)
end

Will that toss an error? Or is the second file appended to the end of the first, making thingus a defined value for the dostuff function?

3. Assuming an addon of considerable size, is it better to have many small files or a few/one large file?

4. Assuming file structure "matters", how does directory structure factor into this?

Thanks in advance!