Often, the DHW is put on a priority zone...this means that when it is calling for heat, the rest of the system does not get any heat (it usually doesn't take all that long to satisfy the indirect - plus, it automatically (normally) ramps the supply temp to max while doing this priority circuit then goes back to modulating to satisfy the house needs). It can circulate what heat's already there, but not add any. Given your situation, the smaller unit should work fine if your numbers are correct. The thing to look at on the indirect is the first hour volume which should be more than enough with a 60 gallon indirect...you could probably go smaller. See what the pros here have to say, but that is my experience. I've got a similar system using a Buderus boiler - it has the priority zone logic built-in, but if not in yours, it is easily added.