It's always been illegal to display replica guns in public. Robbing a store with an airsoft gun (or any other replica) will be considered an armed robbery, ie. just as bad as if you had a real gun.
Airsoft games have to happen in areas where they cause no danger or disturbance to the public. They always require a permit form the land owner. They should always be preannounced to the police to prevent misunderstandings. Biodegradeable BBs are not law-enforced, but since they have become available, increasing numbers of land owners require their use. Usually 8mm guns and guns that require 0,29 or heavier BBs are exceptions, but this always depends on the organizations.
I've never heard of it being illegal to wear a uniform with patches. If it's international law, then so be it, the Finnish police is not aware of it either
Even if somebody would report you they would not do anything about it. It is illegal to disguise as a Finnish government official, be it military, police or other organizations. There are laws regarding public wear of Finnish government medals and other commendations, in practice, you cannot wear them unless you're earned it yourself.
Under 18-year olds can buy guns with a waiver. No usage restrictions apply. Only guns are limited, accessories, bbs and gun parts can be bought freely, though some shops
might interpret this differently.
There are no (law-enforced) power restrictions, nor does the law specify anything about the specific method of action, fire mode or accessories. Full auto is perfectly legal. I cannot think of any accessory that you could use in airsoft that would be illegal. Importing single laser products from outside of EU is not feasible. It is illegal to use non-certified non-tested laser products.
There are no laws regarding the structure or appearance of airsoft guns, though reloading shotgun shells with plastic BBs from shotgun shells does not make the gun airsoft. Airsoft guns can have as much metal as is preferred and they do not need to have warning stickers, orange muzzles or other stupid stuff, even though these sometimes appear with cheap imports originally targeted to the US.
In practice the only real steel gun parts in Finland that you need a permit for are bolt and barrel. Most other parts you can freely adapt to airsoft guns. Obtaining and using a real steel receiver is legal in most cases.
There is no country-wide organization to cover the sport. Area-specific organizations have been making good progress recently and are starting to yield more power. Bigger games are often held by separate entities usually specializing in only bigger events, weekend skirmishes being what local area organizations and sometimes active inviduals put up. There is absolutely no requirement to join any organization to practice the hobby in Finland. Most weekend skirmishing sites are free, most bigger events require participation fee of around 20 euros (could also be quite a bit more or less).
As far as I know, there is only one active commercial field that runs mainly airsoft games.
Airsoft and paintball scenes do not really collaborate much. All the small collaborations I've heard are connected to a single company that specializes in paintball but tries to make some money with airsoft too. I think this is because of the huge difference in average ages between the sports. The average airsofter might be well under 18 in Finland. Paintballers seem to be usually 20-40 years old.