If you have the Steam version of GTA IV, then you more than likely have patch version 22.214.171.124. This is a good thing as all file checks were removed during this patch and you can modify the game openly. So now let me answer your questions one by one.
1. (iceEnhancer) I am not sure why this would have affected your vehicle mod ability, as I am content with the graphics the way they are and have never used ice.
2. In contrary to what Killerfist said, there is a rebuild option in OpenIV. It is only available after you have edited an img file. This option is in the file menu.
3. Yes, if you install too many vehicles you WILL get the Taxi bug. To combat this for normalish traffic flow, you can install the "TrafficLoad" modification. I do believe it is on GTA4-MODS.COM but if it is not, just let me know and I will get you a copy. This mod takes control of the vehicles that spawn around you and regulates it. Also with traffic load, instead of it being random, you can choose which vehicles load around you via an INI file.
4. (Older mods) You just have to read the instructions/readme's and see if they mention a version number that corresponds to the mod. The 126.96.36.199 patch was released over two years ago, so if a mod hasn't been updated prior to March 2011, then that mod may not work with your game.
I also wanted to add a note on custom vehicles. Using custom vehicle skins or editing your handling.dat has been known to make you crash more frequently in online games and can definitely cause other people to crash as well. If you start noticing that you are crashing to the desktop with SMPA60 error, you will have to make a choice. Remove your custom vehicles and play online, suffer the crashes online and keep your vehicles, or remove the custom vehicles and restore your handling.dat.
Please also note that custom vehicles can sometimes be detected as you sometimes appear to be a floating figure online instead of driving around. If the host knows this information, you may be kicked.
Always make backups of your files. Especially vehicles.img and handling.dat as you cannot restore part of the game via steam, but in fact, you have to reinstall the entire game.
Last edited by WaffleHouseTits; 06-07-2013 at 06:52 PM.. Reason: auto-merged double post