Basically, just empty out the IPL files. You may get some game crashes if you don't use a stripped main.scm file.
Make sure you keep the ide files just in case. If the map starts fine with them, remove them one at a time.
You can just edit gta.dat like this -
| CODE |
IDE DATA\MAPS\generic.IDE IDE DATA\MAPS\INDUSTNE\INDUSTNE.ide IDE DATA\MAPS\INDUSTNW\INDUSTNW.ide IDE DATA\MAPS\INDUSTSE\INDUSTSE.ide IDE DATA\MAPS\INDUSTSW\INDUSTSW.ide IDE DATA\MAPS\MAKING\MAKING.ide IDE DATA\MAPS\TEMPPART\TEMPPART.ide IDE DATA\MAPS\INDROADS\INDROADS.ide IDE DATA\MAPS\COMNtop\COMNtop.ide IDE DATA\MAPS\COMNbtm\COMNbtm.ide IDE DATA\MAPS\COMSE\COMSE.ide IDE DATA\MAPS\COMSW\COMSW.ide IDE DATA\MAPS\COMROAD\COMROAD.ide IDE DATA\MAPS\LANDne\LANDne.ide IDE DATA\MAPS\LANDsw\LANDsw.ide IDE DATA\MAPS\SUBROADS\SUBROADS.ide
SPLASH loadsc2 #collision data COLFILE 0 MODELS\COLL\GENERIC.COL COLFILE 1 DATA\MAPS\INDUSTNE\INDUSTNE.col COLFILE 1 DATA\MAPS\INDUSTNW\INDUSTNW.col COLFILE 1 DATA\MAPS\INDUSTSE\INDUSTSE.col COLFILE 1 DATA\MAPS\INDUSTSW\INDUSTSW.col COLFILE 1 DATA\MAPS\MAKING\MAKING.col COLFILE 1 DATA\MAPS\TEMPPART\TEMPPART.col COLFILE 1 DATA\MAPS\INDROADS\INDROADS.col COLFILE 2 DATA\MAPS\COMNtop\COMNtop.col COLFILE 2 DATA\MAPS\COMNbtm\COMNbtm.col COLFILE 2 DATA\MAPS\COMSE\COMSE.col COLFILE 2 DATA\MAPS\COMSW\COMSW.col COLFILE 2 DATA\MAPS\COMROAD\COMROAD.col COLFILE 3 DATA\MAPS\LANDne\LANDne.col COLFILE 3 DATA\MAPS\LANDsw\LANDsw.col COLFILE 3 DATA\MAPS\SUBROADS\SUBROADS.col SPLASH loadsc3 # have to load map.zon before any of the IPLs MAPZONE DATA\MAP.ZON IPL DATA\GTA3.ZON # # Scene information # IPL DATA\MAPS\CULL.IPL |
Or just add a "#" in front of the entries you want it to ignore.

As for adding stuff, just make a new ide file with your creations. Make sure it has a collfile too. Make an IPL in MEd and place them, then save your changes and run the game.