Natasha has bad connotations unfortunately (it's like calling a kid 'gold digger'- I hate the stereotypes and so on and I love the name, but won't name my child that iyswim).
Svetlana is nice. I quite like the nn Lana I think, sounds more English? I like Natalia. Not sure about Tatiana, I like the nn Tanya for it however. My exMIL was called Victoria, so won't use Viktoria. Anastasia is beautiful, I like Ana and Anya actually for a nickname, that would work. I like Katerina and Katya, as Kat is a very strong sounding name imo, and also Elena, but my neighbour is called Elena, which puts me off 
Fr boys- Nikolai- like it. Mikhail isn't really my taste unfortunately. Sasha- well, I like the name Alexandr and it would be a reasonable shortening, and his friends would be able to give him Alex/Xandr as an English nickname too, definitely top of my list right now. I don't really want to Anglicise it with Alexander/adding an e, 
I know an Egor (not an Igor) and I don't really like the name, but thank for the suggestion.
Thanks to everyone 
So far, my favourites are Anastasia, Svetlana, Katerina(Katya) and Elena for girls, and Nikolai and Alexandr for boys.