I think in the core book there are some guide.ines, I don't know the page number. There are also some more expanded rules in UM.
However there's some things you should be able to observe on your own. First in the case of the alchemist why would he want by? He has a class ability that's far better than By and can get discoveries to make it a cone I think if he really wants.
Secondly the Magus already has a built in feature to do this in the form of one of its Magus arcana.
Basically though certain classes have stuff they are supposed to well and stuff they don't do well, when they do get spells that they are weak at its usually higher level. Example clerics getting animate dead at lvl 3 vs wizards at lvl 5.
Anyhow I would avoid it unless its already built into the system like the Magus. The exception being if the new spell is an adventure plot point to overcome an obstacle.
I would allow it as a 1st-level version of fire breath that must be used immediately when imbibed (belch a cone of fire instead of shoot a cone of fire from your hands).
Note that extract formulae must fit with the limitations of the alchemist and are not true spells.
The guidelines in the CRB should work fine. Basically it comes down to whether or not they want to be spending 1000 gp to learn magic missile or not.
As a "new" spell, you should rename it. Fire belch, Force bolt, etc. This stops them from treating the original spell as a spell now on their spell list. So a witch would still require a UMD check to use a wand of magic missile, and there's no chance of buying a wand of force bolt, since no one else knows it.
You should encourage your players to try to work within the existing limits of their classes, though. The section on developing spells explains that it is to create spells that don't emulate existing features and stuff. I think that using it to take spells from other classes' spell lists goes against the intent of the rules.