Serial.writeLn("bla bla bla in quotes") <- I used form this a lot in the main file. Am I reading this correct that this is not a good idea memory-wise?
Correct. Since program memory is not "normally" addressable, all C variables/objects/etc will end up being allocated in RAM, even if their value is constant. In arduino 1.0, steams (xxx.write()) understand "strings allocated in flash", which can be done by adding F("string"), ("Serial.writeln(F("bla bla bla in quotes"))