choosing the suitable focal length really boils down to personal preferences.
what works for me, may do the otherwise for you. i dislike walkabout with 50mm on crop, so there, i say it sucks.
what lens do you have now? maybe you can set the lens(assuming a zoom one) at those focal length which you mentioned and try shooting photos to see if it's workable for you. else you can try renting those lenses from shops.
for the 70-200 f4L, my experience tells me that you will need good light in order for the lens to perform well. go for the IS version if possible. other than that, IQ is stellar, nothing much to complain about.
If you are traveling in a group, people will not keep waiting for you while you change your lens, not to mention you might have to run here and there trying to frame your shot. Are you sure you want fast primes instead of a zoom?
Personally, I still think you might want to consider an ultra wide lens (in addition to your listing of lens) since the 600d has a 1.6x FOV. A wide angle will give you shooting opportunity for landscape regardless of where you are. I know it is a zoom but the EFS 10-22mm is often a good companion. Just watch out for distortion when taking pictures in urban cities.
Applications (my personal style if I were not used the lens that you proposed including the wide angle of the 10-200 suggested) would therefore. be in the following manner:
1) 10-22mm: wide angle application for landscape and urban cityscape.
2) 28mm: street and anything interesting that caught the attention while roaming from places to places. I would also use the 28mm for group photos when the opportunity arises.
3) 50mm: I tend to reserve that for a portrait usage on a single human subject at any one time.
4) 70-200mm: this is a versatile zoom and therefore, I have different usage. Landscape when needed to isolate a certain scene from the rest. I would use it for faces of strangers while on the street. This is because I am not good at approaching for a picture and I prefer the paparazzi style of shooting from afar to capture the natural expression of people going about their daily lives.
I hope this helps and again the above are just some of my personal style of going about in getting the images I need if I were to use the lens as listed in your post. Cheers and enjoy photography. The pictures is what brings the smile to your face.
my personal preference and opinion.
28mm on a crop body will be a little tight for landscape if tts what u intend to use it for.
50mm on a crop body is neither here nor there tts y i sold mine.
70-200 good for street portrait (cos im a little shy so i prefer the distance)