With a small pool of customers, you might want to consider having a blog where they can leave comments instead of a full forum.

Security is the big issue.

You don't want human or robot spammers dropping pharma URLs on your site, or simply clogging up the works, so be sure to have a good registration process, plus captcha at message level.

I'd go for the forum, as you can more easily channel your user requests without necessarily writing anything yourself until a response is needed.

you have to think of a good and efficient way to cave the trolls that might come on the forum

I think a forum is a great tool for a software site. All content is archived and searchable, and you may find that some of your customers start answering questions, too. That can be particularly useful for weird stuff like interactions with other software, etc., that your internal staff might not be familiar with.

Automated spam shouldn't be much of a problem if you use decent software and employ its anti-spam features. It's likely to be low volume for a while, so enable notification of new posts and send the notices to yourself or a tech who's usually around. That will enable quick replies to questions and also allow you to kill any spam or flames quickly.

One final thought. When I consider new software, I always check out the software site's forum. If I see that questions have been replied to quickly and effectively (or, even better, quick answers and lots of helpful posts from users), I buy with confidence. If I see no recent activity, or even worse, questions that have been unanswered for a long time, I move on quickly to the next vendor.

If you are confident about keeping out the riff-raff, then get the best SEO benefit from your forum by adding it to domain.com/forum/, not forum.domain.com, and not domainforum.com

This allows the forum content to be added to the site's own, adding nicely focused content to your total, and makes interlinking a confusion-free doddle, and avoids the need to duplicate marketing and seo effort.

But you do need to be sure that the forum does not add unnecessary risks; you'll need nofollow of very close moderation; you'll need to deny signatures, and you'll need to be reasonably confident that spammers can be kept out.

Not particularly challenging conditions, but vital!

I'm in a similar situation, although with a slightly larger customer base.

I'm looking to do both, blog and forum. The blog will be mostly stories from staff (work related) and news, both about the company and about the various industries our company caters for (around 8).

The forum will be structured in a similar way, with a general (off-topic) section, reseller section, and a staff only section.

My thinking is, the blog will let us start and shape discussion, whereas the forum will allow customers, or prospects, to shape the discussion.

Think I'll start with a blog and announce the forthcoming forum in the blog to raise interest.