Intumescent/cold smoke seal strips come in 10mm, 15mm and 20mm widths. My understanding is that new build/new door and casing calls for 20mm on FD60 doors and that the 10mm and 15mm widths are used only on FD30s or for retrofitting to existing doors (if in doubt talk to the Building Control Officer at your local council). A lot of FD30 door makers seem to prefer the 15mm over the 10mm as it is more certain of reaching the required rating. Generally intumescent/cold smoke seal strips have a depth of 4mm (Edit: Yes, there are 2mm, 3mm, 5mm, etc strips out there, but if you walk into a commercial stockist and ask for them you'll generally get 4mm thick ones......). In your case you are hanging new doors so I'd stick to the standard depth, shallower or deeper grooves are often to accommodate fitting to existing doors which have shrunk and fixing to existing doors which are in some way restricted (e.g. as in a listed building) respectively.
Personally I've never seen a 1/4in shank intumescent strip cutter, although I know that Wealden Tool sell 8mm, 3/8in and 1/2in shank cutters, see here
. You could try Trend or Titman, but I think that they're the same. The alternative is to use an appropriate straight cutter and rout on the surface or to use an appropriate slotting cutter and bearing, which will be available in a 1/4in shank. Both techniques are shown in Trend's Knowledge Base article
. I used that approach before switching to the special purpose 1/2in shank cutters which are faster to set-up as well as faster in use and more accurate. Using a router with the straight cutter on the edge of the door can be particularly "tippy" so a second fence on the opposite side of the door will help stability. Be careful to get the groove width bang on and the depth dead right or you moght find the strip detaching itself from the door as it is opened and closed because of the wiping motion of the brush strip against the door casing.
Before cutting the intumescent grooves you'll need to shoot the door in all round (to fit the casing), but remember that the depth you can cut into a certified fire door is severly limited (often just 3 to 6mm max) and that you are supposed to leave the fire rating label intact and on the top edge if it's a BWF fire rated door (not always possible, that one) or the marker plugs visibe in the hinge edge (for TRADA doors). For a painted door I'd leave a 3mm gap up both sides and at the head with about 6mm at the bottom. Normally a job for the plane, that.
I'm still concerned that your doors aren't proper fire doors, though. 35mm is the thickness of domestic hollow doors which are not fire rated whilst most BWF Certifire or TRADA Q-Mark (the two ratings in use) are as a rule 44mm thick (FD30) or 54mm (FD60). If they need to be fire rated I suggest you check the top edge (BWF) or hinge edge (TRADA) to ensure that they are correctly certified. If on the other hand you are installing the brush strips merely as a draught excluding method fire rating doesn't come into it