"Sila" can be used to politely requesting or giving instruction someone to do something, eg.
Please que up (Sila Beratur). And in this situation, "Sila" and "Tolong" can both be used.
And yes, the correct definition of tolong is to plea for assistance.
Tolong saya dengan masalah ini = Can you help me with this problem .
In this situation tolong can be translated into the word Help, and CANNOT be subsituted with Sila