[Rakol]

Is it possible to modify the models of cars/weapons etc. used when playing using SA-MP but not change the models used while playing without the client (single player)?

It's just I changed the skin/model of the Freeway motorbike and changed some things in handling.cfg so CJ would sit on it right, and then found out I can't do anything with the SA-MP handling.cfg

So can I use separate models/skins for SA-MP than for single player?

[DiDok]

No
Unless you have 2 installments of game on HDD
You can't mod handling, it's always reverted to default in SA:MP