Regarding Coimbra, there is a Centre for Russian Studies in the university.

https://www.uc.pt/fluc/depllc/CER

Here are the Russian courses available.

https://www.uc.pt/fluc/depllc/CER/cercur

There is even a Russian Cinema Club.

https://www.uc.pt/fluc/depllc/CER/kinoklub

So you'd be fine in that regard. I suppose both Porto and Lisbon also have Russian available, but it's just a supposition, I'm not sure. The Coimbra centre seems particularly active and we'll organised though.

Regarding rents Coimbra is undoubtedly the cheapest of the two.

I guess all of them should be OK for someone who wants to practise surf. You can do it all over the west coast of Portugal. From Coimbra, you'd probably go to Figueira da Foz, which has the nearest beaches.

As for the environment, your be fine in any. Coimbra is the smaller city, but it is a student city and lives of the university. It depends on the taste really. But you you'll be good in any of them as far as student experience goes.

I have no experience with part time jobs, so I cannot comment on that.

Finally as fit one year versus six months, all I can say is six months go by really really quick. Again it's personal preference, but having studied abroad myself, I think one year is way better. Of course, it's up to you.

Hope you have a great time :)