QoS on Cisco ASA 5520

I have apporx 200 users on a remote site.
They connect to the central site with two site-to-site ipsec vpns.
All applications are running as published applications on a citrix farm at the central site.
I want enable priority for the traffic going to and from the remote site

I see that it is possible to add a service policy, specify tunnel-group as match criteria and check the "enable priority for this flow" box. That is OK, i can do this for each tunnel-group.

First question:
Under "Configuration - Device Management - Advanced - Priority queue" i have to configure priority queue parameters on a interface. In this scenario, should i use the Outside interface? And i see that the default values are: Queue limit 2048 and Transmission ring limit 512. Is there a rule of thumb or something like when setting theese values?

Second question:
It seemes a bit weird to just put priority on the tunnel-groups, i would really like to specify what i want to put priority on. So, keeping in mind that the traffic is going to and from the central environment over ipsec, is it possible to just define a outside-policy and one class for each tcp/udp port i want to enable priority on?

Third question:
if i am far off, what do you think i should do with my ASA 5520 to implement some kind of QoS for my citrix traffic to and from the remote site?