Hey,
Check the bottom left corner of the Uploads tab, there's an upload slots setting there that's set to 1 by default.
Download priorities work a little differently in SoulseekQt. First, you need to add users to a group. Right click their username in your user list and select 'Set user's groups'. Create a new group if necessary and/or check the group you want the user to be in. Next, you need to set a download priority for that group. When the User List is in view, click the 'Configure User Groups' button at the top. Select the group and set the download priority to higher than 0. You can have multiple groups with different download priorities, and users in those groups will download in that order.