Currently I have set Tixati to add udp://sometracker.com:80 to all new torrents, but I would like to know if there's a way of not adding that tracker to a certain category which is reserved to a private tracker.

So every torrent has udp://sometracker.com:80 added to itself on load, but I need the torrents from that private tracker to be excluded automatically.

How can I achieve this?

I could be wrong (again) but don't think you can have it both ways.

I don't think there is any IF .. then condition that can be applied to the auto-add tracker function.

I think you'll have to decide to either auto-add and manually delete from your private torrents or manually add to the others.

Ur right, currently it's not possible to do so. or at least I haven't found it.

simple if could be private flag on the torrent

Please consider setting this as a priority bug for the next release(s). No tracker should be added to private torrents. This can get users banned.

Uhm.
No tracker can check your scrape data from ANOTHER TRACKER.
Any transfer with the lock icon is already setup as "private" ... there are instances where private torrent swarms exist for defunct trackers. EVEN IF one tracker were able to detect this, what you're describing is not "because I added another tracker"... it's because "i leaked my passkey and there are 100 people connecting to my account".
Separate issue. From what I recall circa oink days, any "private" tracker with these draconian ban rules also has a helpful php button "reset my passkey"
Which works great in your Tixati setup because you feed the old URL, the new URL and it will replace them when you setup the duplicate dialog box and hash the magnet: links.
That forces a re-add of the transfer.