What are the limits for a USB PD charger and device to settle on a charging profile?
The entire intent of Power Delivery specifications is to "provide simple and consistent user experience". The main idea of PD is to provide interoperability and compatibility of all PD-enabled devices, regardless of their manufacturer. The PD devices are supposed to be "un-bundled" from power adapters, this is the whole idea.

Section 10 of PD specifications provides the rules for sources and sinks. The main rule is the advertising of source power capability - PDP, or PD Power, Power Delivery Power. The rest is revolving around, all contracts are based on PDP. So if your sink capability is 65W, but the new source capability is 60 W only, the sink should adapt that if it can. Are there limits? Surely there are, but not to the degree that will affect "user experience". In theory the laptop will likely just charge itself 10% longer, that should be it, since Li-Po batteries can be charged at any current below their maximum.

In short, it should work by definition. 

However, the entire specification is the hell of a mess, electrically and logically, it requires a communication stack nearly as complex as USB itself. Some simple examples of PD contract negotiations contain up to 80 steps to conclude. The specification itself evolves to new substantial revision every two years. I somehow doubt that the environment of such complexity can have uniform interpretations between different product development teams (list of contributing engineers count more than 280 persons!!!), and implementations might have "glitches".