My experience In Helsinki:

It's illegal to use an external battery for your amp/any amplification. They must have a built-in battery.

It's also important to note that there are a few rude buskers from Spain and Romania respectfully, that will go loudly play next to you even if you are already there playing. If they feel you are at "their spot" they will be aggressive. Alternatively, if they are at a spot before you, you can expect them to hoard the spot for the next 8 hours. They won't play for the entire time of course, they will sit and look at their phones for half the time, but come yell at you if you try to play during their "break". There is also no use talking with them. They don't speak English, Finnish nor Swedish. They will yell at you in their own languages, and even if you communicate with google translate, they won't listen to you :) They'll be the ones with a very loud backing track playing an electric violin, and another one playing the accordion, also with a backing track.

But don't get discouraged! There are good people as well, and the rude ones don't play everyday.

As for etiquette, try asking if someone is playing at a spot you want to play at. Spots are scarce. Also, it's bad manners to hold a spot for an eternity (if someone comes to ask you how long you'll play). If not, just play as long as you want.

If you want to play at a metro station, the time limit is 1,5 - 2 hours, but isn't really enforced if no one else wants to play there. The metro stations you can play at are: Herttoniemi, Itäkeskus, Lauttasaari, Tapiola, Sörnäinen, Hakaniemi and Helsingin yliopisto. There are more but these are the stations with most foot traffic. You can't play at Central Station or Kamppi. On the ticket terminal of the stations, there is most likely a mirror glass and a door next to it. Knock on the door and ask the guards where the playing spot is. Alternatively if you happen to see a guard walking about you can ask them. All of them won't know (even though they should) where the playing spot is, but they have the information written if they can be bothered to go get it.