I've always believed that heritage buildings and modern designs (towers) can integrate provided that the tower utilizes design aspects of the older heritage building. That can range from the same materials, colours, window designs and accent features.
The brewery tower for example uses some of these ideas and will fit well with the brewery market.
I think that you can have the Roy Building and a modern tower above and it can work quite well; regardless of how tall you make it or whether it's office or residential. Although personally, given that you are quite close to where the main office buildings are in downtown (with the exception of Maritime Centre); I'd rather see the Roy Building go as offices. It works better with Founder's Square as well. I'd rather see the Discovery centre go residential; given twisted sisters has a residential component and the building with starbucks across the street is apartments.
That being said; this is an example of a hotel development in Calgary that incorporated heritage design and was done in a heritage district. This is the Hyatt Hotel on Centre Street and 8 Avenue. The best shot I could find was from streetview; but the small older heritage buildings on 8 Avenue form part of the elaborate lobby, restaurant, bar and convention centre access. When you walk into that area the lobby is stunning; I think google images has some photos if you put in Hyatt Calgary. But this (IMO) is an example of a heritage development that has a tower but works well - the colour scheme mimics the heritage buildings, plus some of the window design and architectural features are similar (you can't see it that well in the streetview image though).
Hyatt Calgary on Streetview