Thank you for our interest in UI for NativeScript.
At this time you could use some of the available validator listed in the documentation
and unfortunately creating a custom one is not supported.
Regarding some of the mentioned points, about the amount validation, if you would like to set some range for the input you could use RangeValidator. More about it could be found here in the documentation
and in this example
About the validation of two fields, we have already logged a feature request
and this functionality will be available for some of the next versions of the plugin.
I also attached links to some other request about specific validators, If you have further suggestions you could comment on some of them.
In the meantime, if you need to make some custom validation you could read the input from the DataForm with the help of editedObject property. You could also review the example here.
Hope this will help.
Telerik by Progress