I am a bit crazy when it comes to travelling...I think everything is possible or almost.
It depends what type of experience you want! It is your vacation! :)
I just returned from a 2 week trip to Norway, Finland, Denmark, Czech Republic, Slovakia and Latvia.
Of course you can probably see just one or 2 cities per country if you go with your plan A....are you OK with this?
If you want to squeeze in Slovakia and just get a preview of it...Perhaps you can try what I did...
I took the earliest bus out of Prague and came back the same night (left at 7am and came back at 11pm). I felt I got a good overview of Bratislava by walking and sightseeings the city for about 7hours.
(You could take the same bus to Budapest too....I recommend Student Agency. Super bus! Free wifi, free coffee, restrooms, free movies, etc)
I'd probably save Croatia for another time...It is beautiful all year long...but very nice in the summer to enjoy the ocean. And it is a bit further than other countries you have mentioned.
Have a wonderful trip!!! :)