Apart from what's written in the manual. I know a few of them but I was wondering if experienced modders could add the ones they know about, to compile all that information in one place?

The one's I know:

If your editing on a Mac, use a text editor that allows you to change the line ending to Windows format. I use Brackets, which is an open-source one.
Your race name in the schedule file must match the feature race file exactly.
The maximum amount per day is 3 meetings, with 8 races max each
Prep race spelling must match the feature race file and the schedule file.
Race names can't be more than 32 characters maximum
There can't be spaces at the end of the lines in the files, which you can find easily by using a text editor, like Brackets, and choosing "Show Whitespace".

As an added aside, I really recommend using a text editor like Brackets, because it will highlight errors if it can. While editing the featureRace file, if you put the text editor on XML mode, it will show if one of your brackets <> aren't closed properly. Plus it can be set to automatically generate the closing bracket, which helps.

At the moment, I'm motivated by trying to debug a schedule that's crashing to desktop without an error message, but it could help new modders if it was all in one place as well.

A few things on your points, its true that you can only have 3 meetings per day with a maximum of 8 races, however more than 1 of these can be international meetings, in fact all 3 meetings can be without any problems.

Also, there is no limit to the number of prep races now, so you can have more than 10, and you can make 2year old prep races for 3 year old races. Prep races from the previous season work well actually.

Oh, I just assumed because there wasn't any in game like that that I saw, I'll edit the original post and change them

A few questions I don't know the answer to:

Can the data be affected by copying it from a program like Excel?
Which tracks have certain rules, like which terrain you have to use (tracknameX) or that the races have to be in the evening?
If I used every tag from the showcase information, what would be the right order for them, like MONTH PRIZE DISTANCE FORCEFIELD RACECATAG etc

Heres an example of a race data that has some of the tags, I have used the order in which they appear in the instructions PDF, and they seem to work, although the conditions don't seem to work, or at least have a lesser effect if there are two conditions on one race.

<NAME>Gowland & Dawson Nursery<RACECOURSE>Newcastle:cc_dirt1<MONTH>Jan<PRIZE>14000<DISTANCE>8.0<RACECATAG>H85<AGE>2YO<CONDITIONS>maxruns=4<FORCEFIELD>small

I've been copying which is in the featureRace file for the existing races and they have <FORCEFIELD> in the middle, rather than the end. So the order must not be all that import.

This is the order I've been using:

<NAME>Game Stakes<RACECOURSE>Morphettville:cc_flatTurf1
<MONTH>Mar<PRIZE>175000<FORCEFIELD>large<EXTFIELDSPEC>home<DISTANCE>9.0<RACECATAG>H65<AGE>2YO<SEX>CG<CONDITIONS>maxruns=0

I've never actually applied force-field and home at the same time though, I've just set up a template that has them both and applies the right one based on the race data.