It wouldn't be wise to fit a full off road tyre at one end a full road sports tyre at the rear, but there's absolutely nothing saying you can't. If it was an issue there'd be warnings on the tyre walls, but there's actually much less difference between brands and types than most people make a fuss about, compounds are within a fairly tight range and other than block style off road tyres, most tread patterns are devised by the marketing department. If you load up the rear of the bike to go on a big trip you change the balance of the bike completely, nobody seems to rush out for a different rear tyre though.
