Suggested

forced to use soft and hard tyres in races

I have just been beaten by a guy who used SS tyres in all pitstops because I was sticking to current real rules meaning you must use both compounds of tyres.

Firstly I couldn't find it explained anywhere that I could just use super softs all the way through, and secondly, this is a GREAT feature in the real F1 races adding extra strategy due to the loss of lap time.

A feature you plan to add or should I just stick to using the best tyres throughout?

For now you should stick with what you think is the best combination of tyres for the track and race length given, of course.

Another thing I noticed: You don't have to pit even though the lowest amounts of stops you can choose is 1. But you can give the first stint enough fuel to complete the race, at least in the shorter race leagues. But even in 50% races it's usually not a good tactic though, the weight of fuel usually slows you down way more than you would loose by a pit stop.

Game rules and full manual isn't ready yet. Usually it doesn't makes sense to do one until a game reaches a state where the foundations are good and the changes are fixes, tweaks and expansions. Else it costs too much time to keep it updated and most of the time it still ends being wildly outdated.

I suppose having a limited amount of compounds or the complete real rules would only really work with 100% races. It does sound interesting though, but since it severly limits choices it easily could mean there's only one working template you'll have to use.
In real F1 the thing is that overtaking is a risk that should be avoided, or downright nearly impossible, and thus things like where in the field you end up when is often way more important than which choice is making you go faster. Thus limitiations that increase the difference between cars at certain points into a race could make it more interesting. In iGP overtaking is quite easy and the only thing that really counts is the choice that makes you get to the finish line the quickest. So choice limitations might help the emergence of a single best choice template you have to use if you want to win.

How would it be implemented? You can change your pit strategy during the race, so even if you have some kind of check when the player choosing tyres pre-race, they could still change their strategy on race day.

I can think of two ways to implement this; by disqualifying any finishers that have only used one compound throughout the race or.... by doing a check before the final lap, and if a driver has only used one compound up to that point, forcing the driver to pit and change tyres. The latter means that even if you forget, the worse that can happen is you lose a 'pit stop', and not get disqualified.

The way around that would be to Qualify and do 2 laps on SS with the highest push level then get DRS & KERS the next lap around on the harder compound.

Rules for leagues should be in the league forum page and the league host should be notifying all new comers to the league in the league chat of what the rules are, if there are any. Most if not all leagues let you do what want.

A 2 compound rule can't work within the game because you can't change the push level for each stint.

We tried it in a league I was in and it did improve the racing and vary the strategies but those who couldn't attend the live race were severely handicapped so it was dropped.

Perhaps it is worth revisiting when the mobile app is released. I'm amazed your league mates did it voluntarily!