I was going to post this in the other thread in response to the question there but as I typed out the post and it got longer and broader it seemed better placed here.
I definitely think the Valdor should be in the Skitiarii list
The Ad-Mech are less known than other armies so there's more guesswork / filling in the blanks needed and list writing for them can be taken in a number of different directions. There are some old background materials for them, but mentions in recent background as to the Ad-Mech are particularly important as they show us the direction FW/GW is taking them, as FW will very likely do an Ad-Mech list eventually.
The background for the Valdor talks about it being rare, old, difficult to maintain technology and that it is most often found in Ad-Mech armies these days. With the Vraks Campaign originally billed to feature the Ad-Mech besieging the Citadel towards the end of the war (it was decided there wasn't space/time to do them properly when the campaign was shortened to only 3 books and so they were left for a future time instead), it is possible the Valdor may even have been conceived and designed to be included with the Ad-Mech originally, before being included as old stored tanks raided by the heretics instead.
A design theme of Ad-Mech armies is that they use a greater proportion of old higher technology items and vehicles than other armies, things which are not around much in other imperial armies anymore because they are hard to maintain and/or the knowledge of how to do so has been lost, but the Ad-Mech are the experts at repairing and maintaining ancient technology and the Valdor fits very well with this.
Another very appropriate choice for the list, given these criteria, would be the Minotaur as this is described an extremely ancient design (possibly the oldest tank design still used) and one that is very high tech and harder to maintain. It'd be very appropriate for the Ordo Reductor who are the feared siege specialists branch of the Ad-Mech.
One more idea I have to emphasise the fact Ad-Mech make and repair the technology and keep more of the elite better stuff for themselves could be to allow 0-2 Vanquisher / Executioner upgrades in a Leman Russ company and/or allow 0-1 in a Leman Russ upgrade. Back in the day there did used to be whole companies of Vanquishers before they became rare.
If the Valdor is accepted into the list the other similar smaller unit it would make sense to include is the Leman Russ Tank Destroyer. At the moment the list seems too similar to vanilla IG with mostly a liberal sprinkling of new War Engine unit choices, but there's too much emphasis on the big stuff but too little smaller tech vehicles, which there should be too and which this would correct. These could be represented as separate units or possibly having Destroyer units with a Valdor command tank could be an idea?
I have lots of other thoughts and suggestions for the Ad-Mech list, which looks decent now but could be improved and made more characterful/interesting IMO, but I'm slowly thinking through the rest and will post my thoughts up in time (maybe not till new year till once Siege's finished as this was when E&C mentioned going back to the Ad-Mech then and better to concentrate on Krieg first). I'm actually painting up an Ad-Mech army for use in the WW tournament at the end of January so am particularly Ad-Mech keen these days
