Yes. Are you sure it allows 26? A post I've read said 24.
If you hover over the track descriptions the image shows how many grid spots, All official tracks are 24 except Magione 18, Vallelunga 22, Monza old 25 and Monza new 26.
26 Cars at Monza may be quite a system killer for guys on lower hardware but not much worse then 24 I guess.
You should be able to add 26 into the practice server at least, even when you added 29 it was only the last 3 cars not in the pits but hovering somewhere in the infield.