Is it acceptable to require a post-doc to teach a course?
In the UK universities, the pressure to teach is very significant. Postdocs employed on RCUK grants / fellowships are often given an "opportunity" to contribute 20% of their time to teaching. This is not a formal requirement -- they can, of course, reject the opportunity and focus on research. But focusing 100% on research would not necessarily be wise, since their next University is likely to look for candidates with some teaching experience already (unless they move to a different country / sector).

Since it is your fellowship, you can set your own rules to a certain extent. I have heard of PhD studentships with attached duties, e.g. to staff a University museum for a week, etc. It is not unusual for a fellowship / bursary / studentship to come with some strings attached, and it is may be even a good thing to help selecting "right" candidates for the fellowship.