I've heard lavender can be quite aggressive - will it choke out the fuscia?

There are lots of different types of lavender. There are dwarf varieties
which will not get that big, French ones (the ones with little "tufty bits"
at teh top of the flowers) which do not tend to like it to get too cold in
the winter and "normal" ones. You can prune all of them by cutting them back
after they have flowered but do not cut into the dead wood (i.e. the bottoms
of the stalks where there are no fresh leaves) as these will never regroup.
Hayley