Hi Christian,
Thank you for your interest in Syncfusion products.
1. Is it possible to use a custom control in the gallery?
It is not possible to add the custom control in the gallery. Mainly it is to add the image collection. You can use the ToolStripEx to add the custom control.
Please refer the below code snippet:
ToolStripControlHost host1 = new ToolStripControlHost(this.treeViewAdv1);
this.toolStripEx1.Items.Add(host1);
You can find the sample from the sample browser
\\Syncfusion\EssentialStudio\5.2.0.25\Windows\Tools.Windows\Samples\2.0\Office2007 Controls\RibbonControlAdvDemo
2. I could use a dropdown button with a popup window but a compact stroller would be better
If the control size is bigger than the ToolStripEx size, then the dropdown button will be shown to display the control. If you don't want this, you can make the control size as ToolstripEx size. Now it will not show the dropdown button, it will show the control in the toolStripEx area.
Please download the sample from the sample below link:
http://websamples.syncfusion.com/samples/Tools.Windows/F69920/main.htmPlease let me know if you have any other queries.
Best Regards,
Bhuvana