Custom Fields - Type - User Groups

[joomleb]

Hi guys,
Joomla 3.7.4, creating a new Custom Field - Type - User Groups, looking to have a User field where to select "No/Yes" to add User to another User Group (in teory moving him from "Registered" 2 to "Associated" 10). So:

Default User Groups (A comma separated list of user groups ids): 2, 10
Multiple: No

BUG ? "Default" can be only one ?!? No ?
BUG ? Is shown as default always the highest Group ID
BUG ? Always is shown the entire User Groups list, I cannot limit the User Groups. Too dangerous, a user can be wrongly added to another user group, also as "Super User"
BUG ? Also setting "Multiple" to "Yes" the two user Groups are prefilled, but I can always select other User Groups from the list (too dangerous).

I suppose the two fields have to be modified and a field is missing, like:

Default User Group (only one): 2
User Groups (A comma separated list of user groups ids): 2, 10 (the only User Groups shown)
View (Multiple): ... Should have to be changed as "View", where to select from: Multiple No / Multiple Yes / List No/Yes, where for each listed User Group I can select No/Yes

Please, Can you confirm ?
Do you have any ideas / suggestions about it ?

Many thanks for participating