An continuously indexing POA might be a bit much to share space with, but almost anything else is good. Guinevere only takes about 13 MB of disk space. Of course, the main drain on this space will be the Work Directory, where files coming through the gateway are decoded, decompressed, and scanned. The amount of space required there really depends on the kind of traffic you receive. You really only need the command line scanner portion of your product plus any virus signature files.
Some vendors offer multiple command line scanners.
If the working directory is on the network, be sure to exclude this directory and all subdirectories from scanning by a server-based scanner. Three reasons, in decreasing order of lameness. First, I don't know C that well, and NLMS sound pretty nasty to learn. Java would be a possibility, but would narrow the market to NW 4.11 and 5 servers. That strikes me as just too darn boring.