can i only use the service for one month

You can upgrade for one month and then downgrade back

If you upgrade for one month and then downgrade a couple of days later the 200GB will continue until the end of the subscription month before downgrading. Don't leave downgrading until the last day or it may not 'take'.

Also remember when you down-grade, Apple will end up purging any data beyond what fits into whatever you downgrade to (50GB or the free 5GB). So if there is anything you want to keep, transfer it somewhere else for safekeeping before the downgrade goes into effect.

One way to save your data.

Archive or make copies of your iCloud data