I have never tried to work in a company where we have been 100% Agile, there is always someone who are not working with Agile methods and very often do not know about Agile.
There is a huge difference in the impact of others not working with Agile, it is my experience that it very much depends on the support from higher level management.
I have worked in a company where the decission to go Agile came from the top level management and I have worked in companies where we started to work with Agile as a bottom up idea, i.e. without higher level management support.
When I worked at Nokia, we implemented Agile on request from higher level management, this gave us mandate and commitment to work with Agile, in the beginning we did very often meet people who did not know about Agile/Scum (other than what Google give you), but they were always positive to learn and explore the new way of working.
I once worked in a company were we was using both Agile and traditional water fall methods, our team was working with Agile, however the product program was still using Microsoft project, and we had to give them feedback about our plans, deliveries etc. - they did not accept a burndown based on the teams velocity, so our SM had to use his Sunday to convert our backlog and burndown to a Microsoft project document...
I was participating in a seminar about Agile development methods - in practice it was about Scrum - and we discussed the success rate for implementing Agile bottom up rather than top down and there wasn't anyone who had a positive story about bottom up success.... :-(
My recommendation is to get higher level management to approve that the organisation work with Agile methods to avoid too many other challenges.