commercial 009 gauge models of 3' gauge prototypes 4mm scale or a smaller scale (eg 2.5mm) that will fit in with other 4mm 009 stock please?
I all depends on what other locos and rolling stock you have. There are some that are tiny and others that are massive but are all the same scale. One thing to look for is the size of the people on your layout compared to the doorways and roof heights.
Is that a typo and meant to be 3.5 mm instead of 2.5 mm?
If so, a debate crops up on occasion on mixing 3.5 mm (i.e. H0e) and 4 mm (i.e. 009) stock. The main consideration is relative sizes of doors, steps and other things that reflect the size of a person, but for goods stock it can be less noticable. Also 'continental' H0e models may be of larger prototypes compared to smaller prototypes that may be more typical of the UK NG railways generally modelled in 009. There are some prototypes where it is barely possible to stand upright and there are others where there is adequate headroom.
Also, some people modify TT scale models, particularly old Triang ones to turn them into 009. One example is tanker wagons. (TT scale comes in different versions - 3 mm to the foot (about 1:100) or 1:120 depending on era and country and there is also 3 mm finescale, but I'm on shaky ground here as I have no direct experience of this range of scales.)
I use several 3mm scale 'TT' wagons on my H0e layouts, but only open wagons and tank wagons, not closed vans. I also have several locos that use 'TT' 3mm scale bodies. The recent modifications to one of these locos, to make it look better, will be in 009 News in the not too distant future!
I think the 1st point to make is that when it comes to narrow gauge there isn't much of a standard...
The main thing is whilst the track gauge may be the same the loading gauge - essentially how big the stock is - can be quite different. A good illustration of this is standing on the platform at Porthmadoc and comparing the size of the NGG16 garratt and it's WHR train on one side to the much smaller FR loco and stock on the other line - but the track gauge is the same.
When it comes to the 3' gauge lines on the island of Ireland the loading gauge is smallest in the south - Schull & Sibbereen - and gets bigger as you move north with the County Donegal and Lough Swilly lines at the other end of the size range.
All of the plastic kits easily available in the UK for 3' gauge stock are to 4mm scale (to the best of my knowledge):
For 009 use and keeping to 4mm scale the County Donegal stock is possibly a bit large (Nine Lines used to make the plastic kits that were taken on by N-Drive Productions). The Tralee & Dingle & West Clare stock (kits by Dundas & 009 Society) look OK on 009 if you are modelling something 'large Colonial NG' in style with a 2' 6" gauge, and whilst there aren't any kits that come to mind for the S&S it's probably not to far off from the Southwold Railway(a 3' gauge lines in England). These kits, like the Donegal stock, used to be made by Nine Lines & were taken on by N-Drive and sit quite well alongside Welshpool & Llanfair stock.
As Brian suggests it will depend on your other stock as to how well the 3' gauge stock will look, but I'd suggest that if (for example) you have some Peco FR coaches I doubt if it will work. It might if you have some Ashover coaches and WD stock, but maybe not if you add Donegal vans & wagons.
Edit: Having come back and re-read the above I've discounted any kits for HOn3. These are generally for US narrow gauge, and their 3' gauge lines had (have) quite a big loading gauge, probably more like the County Donegal. That being said as they are to HO scale (so 1:87 rather than 1:76) they come out slightly smaller.
I have a few bogie open wagons that I've fitted with 9 mm gauge bogies that I look good with my other 'large colonial' 009 stock, but the issue with vans is that they can look a little short in height, especially compared with human figures. It's probably a personal thing, but when you consider that most goods vans would be designed that a 6' tall person could walk around inside (to load / unload the van) the HO scale version is sized for a 21mm tall figure, and in my view don't really work when run alongside 4mm scale stock. I tend to avoid using HO scale coaches for much the same reasons.
I think the 3 experts who have replied have given a very comprehensive response to your question, I would just supplement this slightly with my own contribution.
I have old Triang TT wagon and tank bodies that run on N chassis (The Peco N scale 10ft and 15ft wagon chassis kits are ideal), I've also used TT loco bodies as a basis for loco building onto N loco chassis. Already covered - the Dundas T&D vans plus the Society exclusive cattle van all suit 009 very well as long as you don't mix them with small loading gauge North Wales stock (ie Ffestiniog and Talyllyn etc) - A train with this type of stock does not fit well with a Bachmann Hunslet including Linda and Blanche - how do I know?...............
For my own 009 modelling preference, I model mainly freelance "inspired by but not replicas of real stock" and have standardised on a "general" loading gauge of 10ft high from the rail top and 7ft wide. (ie 4mm scale = 40mm x 28mm). I say "general" as things like coach roof lamp tops and guards van duckets are often "slightly" over the standard laoading gauge!
I'd recommend having a look at Ted Polet's Craigcorrie & Dunalistair Railway - Google "Dutch Group 009 Society" https://www.009dutch.nl/ scroll to "projects" then scroll to "Ted Polet", allocate at least 1 evening to reading through his entries and I think all your queries will be answered!
Happy Modelling
MickT