Hi, if you don't mind, I'll divide your questuions on two: First - Is it easy to get to Sheremetyevo at 7.15 a.m. or not. First of all there are three terminals called Sheremetyevo: I think that you mean the terminal 2 Sheremetyevo, as it is the ordinary airport for the internationals flights, but you should to make it clear, even if you call the taxi you should know it. There are trains from the center of the city the Savelovsky gare ( to get there you could use metro), but the first train is at 6.45 a.m. and the last at 23.55 p.m. It takes about 40 minutes to get to the airport (all terminals one by one) and it costs 250 rub (7 euro). If the flight is from terminal two, than there are a lot of cafes and shops some of them 24 hours opened, but in others terminals I'm not sure. If you call taxi it may cost about 1000-2000 rub or 25-50 euro depends on taxi.
Second: If you have only two weeks and half it will be very hard to get to many different places and may be it is not very rational idea to get to the cities, that are far from Moscow and S.-Petersburg, but there are a lot of small towns that around Moscow (Tula, Orel, Vladimir, Suzdal) or country estates in the neighbourhood (some of them are the museams now like Melihovo, some of them are in the city's areas like Tsaritsyno or Kolomenskoe) or around S.Petersburg there are Pavlovsk, Tsarskoe Selo, Peterhof (there are a lot of gardens, museums fountains especially in Peterhof), also you could get from S. Petersburg to Krondstadt by train or by sea it's very interesting too. There is one little thing, that you should know - in June-July threre are The Whight Nights in S.Petersburg and there are a lot of people there. So if you want to get into a not expensive hotel, that is situated in city center you should book it more than a month before or be very lucky.
Good luck