Those people were blunderers who had no idea what they were doing.
Indeed. Many will have seen where it says "63A" on the front of the RCD and (incorrectly) assumed that it was an overcurrent trip rating. It's one of the first things I need to fix when (if !) we move as the vendor (or rather, his father in law) made exactly this assumption.
The plan was to use 30mA RCDs in the workshop, and a 100mA at the supply.
And guess what, when one trips, you'll almost certainly find that the other does as well
The reason for that is because when you have a fault, it is very unlikely that it will only be between 30mA and 100mA. In practice, it is likely to be significantly higher than 100mA and will trip both RCDs. It is a false assumption that RCDs somehow limit the actual fault current.
Where it is necessary to have multiple RCDs in series, then they are normally configured with time discrimination. For example, there might be a 30mA/30ms RCD on the final circuit, and a 100ms RCD upstream - that way, if the fault is on the final circuit it will trip the downstream RCD and remove the fault before the upstream RCD trips.
Whether an RCD with 100ms trip time is appropriate to a circuit is a matter of designing correctly while armed with detailed knowledge.
In general you would be well adviced to try and design (cable type and installation methods) the circuit such that RCD protection of the submain cable is not required - that way there's no RCD in the house to have to go and reset when the garage lights go out. Put the RCD protection in the garage where it's easy to get to when it needs resetting.
As to what rating of MCB (or RCBO)/Fuse you need - that can only be determined after you have designed the circuit. That design will include (not exclusively) : ensuring that the protection is appropriate for the size/type/installation method of the cable; and ensuring that the prospective fault current will be sufficient to ensure that required disconnect times are met.