I am developing the plugin with PHP 5.4 on Joomla 2.5 with VirtueMart 2.0, as well as on Joomla 3.3 with VirtueMart 2.9.9.
Other combinations might work, but I have not had any reports. If you are using the plugin on a different combination of versions, please let me know whether it works or not.
Unfortunately, the shipping code of VirtueMart 1.1.x and 2.x is completely different, so a plugin for VirtueMart 1.1.x would essentially be a newly written plugin. As support for VM 1.x has officially ended, I have no plans to write a plugin for an outdated VM version (even though I completely understand the users that are stuck with VM 1 and would need a nice solution like my plugin. It is just not worth the effort to me.
Note however, that the plugin is distributed under the GPL, so if anyone has the time to port it to VM 1, feel free to take on this project.
See the documentation for a full list.
You can't. This is not the aim of this shipping plugin. With this plugin you can not distinguish particular products in the cart, you can only use maximum/minimum and total values in the cart, irrespective of which products cause these values.
However, there are several shipping plugins available that are tailored to your situation. See the JED.
No, I'm not aware of any incompatibilities. If you encounter any problems, please tell me.
I'm open for suggestions, and if time allows, I will implement good new features. Notice, however, that my time is very scarce. Here is a list of things that are already on my list:
First, you need to have a clear vision how to describe the conditions for a certain shipping cost as a mathematical formulat. Once you can describe the conditions as a formula, usually it's straigthforward to write a rule.