Windows

1) no effect on windows, other than less drive space available to windows [ the space used by solaris ]

2) no problems, unless you create a document in solaris and forget to copy it to the windows partition, it would only be available to the solaris installation then.
[ Microsoft doesn't play nice about reading other filesystems ]

3) malware? what's that? [ for the solaris install ] oh, you mean the weakness of microsoft crap, that doesn't affect any other os.

4) install type? workstation for using as a workstation, server for use as server. you can install everything, it would only take about 8 gigs of drive space, but you wouldn't use 95% of what's installed. It wouldn't hurt to install everything, then if you don't like a specific application you can remove it.

5) windows requires more memory than solaris.
the only reason to install more memory would be to work with solaris as a virtual machine, in which case, yup, windows needs the memory to handle the workload.

the pro of installing a dual boot, you have the opportunity to learn a unix os.
the cons, you'll still use windows by default, and rarely use the other os to learn it.

Yes they can coexist but i would not recommend you to do that because it would introduce alot of more complication to your computer.