I saw that you can set games as pre-releases (betas) of other games like the Crash Bandicoot Beta does: I tried to do this for the Beta of Prince of Persia: The Sands of Time aswell ( http://www.speedrun.com/pop_sot_beta ), but the pre-release-option is greyed out and I can't click on it (as can be seen here) Hope, someone can help me with that :)

I am pretty sure there is a bug for "game types" since the new version of the site and that these infos must be hardcoded manually if you want to change it. Just be patient :)

I can't find this game from your link or the search bar. It's not a bug, we intentionally don't let mods attach to random games right now, so site staff has to handle it. Ideally you would be able to attach to a game if you mod it. (However, the specific part of that action of assigning the parent game is actually broken last I checked.)

Okay, thanks for your answer! (the pop community decided that an own page for that beta demo doesn't make much sense so I deleted it, still good to know for future references :) )