How to not have customers in the /rest/api/3/groupuserpicker endpoint?

I try to scale up a connect app and have one global user search function. I hit the problem, that the endpoint /rest/api/3/groupuserpicker can exclude apps, but not customers from the search. So I have to filter them out after the request. I set a limit of users with maxResults, but this is including users of accountType ‘customer’. And if a company has many customers I may loose some ‘atlassian’ accountType users, because they are not returned because of the limit.

How can I say read 100 users with a search query of the accountType ‘atlassian’ using this endpoint?

And: where is the max for maxResults? For normal user searches it seems to be 1000.