Kaikki olivat suunnitelmaa vastaan.

Because kaikki here refers to all the people / everyone, so the verb is plural in standard Finnish.

• kaikki olivat = everyone was / all were
• olivat is the 3rd person plural form of olla (to be)

Even though English often uses everyone was with a singular-looking subject, Finnish treats this idea as plural here.

Kaikki means everyone or all (of them) here.

Its basic meaning is all / everything / everyone, but the exact translation depends on context:

• Kaikki olivat... = Everyone was... / All of them were...
• If the context is about people, kaikki usually means everyone.
• If the context is about things, it can mean everything or all of them.

In this sentence, because of olivat and the idea of being against the plan, it most naturally means everyone.

Because vastaan is a postposition that requires the noun before it to be in the partitive case.

So:

• suunnitelma = plan (basic form)
• suunnitelmaa = plan in the partitive singular

That is why Finnish says:

• suunnitelmaa vastaan = against the plan

This is something you mostly learn as a fixed pattern:

• jotakin vastaan = against something
• häntä vastaan = against him/her
• sotaa vastaan = against war

It is a postposition.

In English, prepositions usually come before the noun:

• against the plan

In Finnish, vastaan comes after the noun:

• suunnitelmaa vastaan

So it works like this:

• noun in partitive + vastaan

That is why it is called a postposition rather than a preposition.

Yes, very much so.

X:ää vastaan is a common Finnish structure meaning against X.

Examples:

• lakia vastaan = against the law
• hallitusta vastaan = against the government
• ehdotusta vastaan = against the proposal

So in your sentence, suunnitelmaa vastaan is a normal and natural way to say against the plan.

Yes. Finnish word order is fairly flexible.

The neutral order here is:

• Kaikki olivat suunnitelmaa vastaan.

But you could also say:

• Suunnitelmaa vastaan kaikki olivat.

This changes the emphasis. The second version highlights the plan or the idea of being against it more strongly. It can sound marked or stylistic depending on context.

For basic learning, the original word order is the best one to use.

Because Finnish commonly expresses this idea with olla ... vastaan:

• olla jotakin vastaan = to be against something

So the sentence is built literally like:

• All were against the plan

This is the normal way to express opposition to something in Finnish.

Grammatically, kaikki can sometimes mean everything, but in this sentence that reading is unlikely.

Why?

• olivat is plural and strongly suggests a group, especially people
• being against a plan is something people are typically described as doing

So the natural interpretation is:

• Everyone was against the plan
• or All of them were against the plan

The negative version would be:

• Kaikki eivät olleet suunnitelmaa vastaan.

This means:

• Not everyone was against the plan.

Notice that Finnish negation uses:

• eivät = negative verb for 3rd person plural
• olleet = past participle form used in the negative past

So:

• Kaikki olivat... = Everyone was...
• Kaikki eivät olleet... = Not everyone was...

Be careful: this does not mean No one was against the plan.
For that, you would say:

• Kukaan ei ollut suunnitelmaa vastaan.

It is in the past tense.

• ovat = are
• olivat = were

So:

• Kaikki ovat suunnitelmaa vastaan. = Everyone is against the plan.
• Kaikki olivat suunnitelmaa vastaan. = Everyone was against the plan.