Without doing the math on the actual heat loads and thermal masses of each zone and the modulation range of the boiler it's hard to know if any of it makes any sense. I haven't measured the thermal mass and couldn't find volume specs of the Runtals, but I suspect they're considerably more massive (think "water volume", not dry-weight) than fin-tube (cood b rong offen am.)
Electric HW tanks can be used as buffer tanks, but whether you need one at all isn't really obvious, but since it's microzoned with TRVs that are each agnostic of the state of the others it may be necessary to prevent short-cycling (or not- it depends). It need not be plumbed as a hydraulic separator- you can plumb it in series, but if your system is already primary/secondary using the buffer as the point of hydraulic separation the mass of the tank is involved with every burn.
Whether 10 gallons would be enough also depends. eg: Assuming all TRV are off and there's ~5 gallons of water in the bypass plumbing & boiler, 10 in the buffer, you're talking ~125lbs of water. If the boiler's min-mod output is 15KBTU/hr (250BTU/min) and they boiler's hysteresis is 5F you get a 2.5 minute minimum burn, which is on the short side but still fine for a smallest in class low mass boiler. If min/mod is 25K (416BTU/min) and the hysteresis is 3F the min burn is reduced to less than a minute- it's a short cycle, and even 20 gallons would fall short. (Ideally every burn would be 5+ minutes, but 2.5-ish isn't going to short cycle it into low efficiency and an early grave the way it would a cast iron boiler.)
The buffer doesn't need a sacrificial anode since it's a closed system, but SFAIK it's not a problem to leave it in.