Most tracks work fine in most of the "slots". Still there is some
difference between each slot (hardcoded in gp2.exe) in terms of:
nominal length of the track, as used to calculate average speeds
the number of laps to race in a full-grandprix
AI-pitentry/exit behaviour (when they brake, and choose another line)
laptime-corrector-factor
For these reasons, the creator usually advises one slot or another.
"Problems" or "weird behaviour" can be absent, or hard to spot (sometimes
AI crash into the pitwall or slow down way too early... but you will only
see that if you do a long race)
Apart from the "normal" tracks and slots, there are a few other strange
things. If you place any track in slot-16, you will see the AI is three
seconds slower than anywhere else. If you place the add-on Zandvoort-2000
track in slot 2, you will see the road disappear temporarily before s/f,
while this doesnt happen in slot 1.
In conclusion: Most tracks you can move around wherever you want, although
there may be slight differences in every slot. A few tracks are really
picky about the slot. Best bet is to try.
As to Monaco, that one is likely to be picky. I wouldnt advice to move a
monaco-based track to anything but slot 4. But you can always try :-)
artijn