My Partner used to be a pro wedding cake maker and I've helped her do a few over the years for friends and family.
The tiers can be any cake these days, but only a fruit cake is dense enough to support tiers above it without dowels, use of a cake stand neatly gets around this.
It used to be a tradition that the top (smallest) tier would be a fruit cake that would be kept until the christening or birth of the first child. For the cake to keep it would need to be a fruit cake and carefully iced so that it is sealed to the cake board to keep air out.
If tradition is important then also consider royal icing, hard work and most people would be surprised to hear that the cake had been sanded down between layers!
I know the Delia recipe that you are talking of and have made it numerous times because it tastes great :-) use light muscavado sugar to keep the taste of treacle from being too strong. It scales up with no problem at all but fruit cake mixture is hard work to stir be careful not to overload even a strong mixer or your arm.
Another thing that wedding guests often don't realise is that there is far more cake than they saw being cut; It is really common for slabs of cake to be supplied, they will be iced to match the displayed cake but without decoration. This way the tiered cake could be all fruit but there can be many other types / flavours of cake for the guests who because of the icing never knew what was in the tiers.