@giubenn, There won't be any issues going back to the 0 or 1 cardinality link. You will have to remove the current link that you have and create a new one. Make sure you identify the workflows and stop them before making any changes to the link type. Also, having 0 or 1 cardinality may affect performance. Take a look at the data model best practices documentation.
Now, when you use the custom resource (ex. transactions) with an N cardinality collection link, you don't know how many records (transactions) are associated with the particular profile. Usually, you will have to identify upfront in the workflows on how many linked records (transactions) you want to display in the message will give you the option to choose within an email designer to choose the data to display.
Thanks, Sathees