Soviet wrote:fuck.
Don't let me put you off trying.
What I would recommend is that you open up the basic gametype files (dm/tdm) and go through it line by line making notes on what every thing is doing. This can be time consuming but it will show you what you know and what you do not know.
If theres anything you dont know, just ask.
Then once you have got through the DM or TDM files, you may want to look into a harder gametype. S&D is the very hardest, there are many trhings going on at once, and trying to keep track of things is very hard. CTF is a lot more simple, but does go into detail about using "think" threads (a thread that is constantly thinking

).
Once you have got to grips with it, start doing some crazy things. Doesn't have to be anything special, but just do different things to test yourself...
for example, make some Cvars that dont exist (e.g scr_ctf_flagdroptime), you will find somewhere in the ctf.gsc that the flag is on the floor for 120 seconds, there is no way of changing this...
Things like that are what get you started.
------------
About your ideas though, a game where TDM is mixed with CTF, is CTF, to combine them would not be to big of a difference... unless of course i'm not exactly on your wave length about combining the 2...
Also, you must remember, not all gametype scripting results in a new gametype, just a lot of changes to how a gametype is played (RSD - reinforced search and destroy, S&D with respawns and must blow up both objectives)...
------------
A little off the current topic, I have restarted scripting for Base Race. The first time I attempted it I failed miserably, but this time I have cracked it
I currently have a multiple objective CTF (2 flags per team). All I have to do now is to tell the script which construction the construction kits are returned to and then tell the game to upgrade the required feature... (which shouldn't be too hard)...
If anyone would like to help, I need a few things done... mainly iwi files (icons and textures)... More details soon
