If your DNO fuse is 60 amp, then 63 amp RCD's are large enough.
We look at diversity, in that not all circuits are at max all the time, and to date I have been lucky, I have two electric showers, and the DNO fuse has not ruptured to date, I do have a battery that can deliver 3 kW and solar panels which can deliver if conditions are right 5 kW, so my total is just under 100 amp, but if I had the pipe work at the showers, I would convert to thermostatic mixer showers, they are far better as
@securespark says, and it would take away the worry that my daughter and wife will both use showers at the same time.
The RCD question is a little more complex to what one first thinks, the problem today with so many switch mode power supplies, is a type AC RCD may not work due to some faulty equipment allowing DC to freeze the RCD, even the type A is only good for 6 mA, so the more RCD's the better, as it reduces the chance of some item stopping a RCD working, and it also reduces the chance or a build up of background leakage tripping a RCD when there is no real fault, so we have moved to favour RCBO's which are a RCD and MCB combined. This means you have extra slots with the same length of consumer unit, some taken up with the SPD, but likely you will gain slots with a consumer unit upgrade.
In the main, people go for electric showers as cheaper than the hot and cold supply showers, however by time one has swapped the consumer unit, and laid the 10 mm² cable, I would think a hot and cold feed shower will price not much differently to an electric heated one, of course can still use electric to drive shower pumps, but this only needs a 5 amp supply, a FCU off the ring final is likely enough.
If you have a combi boiler, likely no need for a pump, also showers downstairs don't need a pump. But I know I liked my mother's power shower, had to remove it when combi boiler fitted.
